The following set of instructions appeared on the KappaPerformance forum and were written by DeepBlueGxp. Thanks Joe.

The seat heat pads installed were purchased from

http://www.sportsimportsltd.com/pecafidutewa.html

Make sure you purchase the Add A circuit or you can buy one locally.

I've added some supplemental pictures provided by JPM, Elff, TomatoSoup, 2WK4U and some additional comments. I want to thank these gentlemen for the excellent work they did and for documenting this modification as well as they did. The only reason for putting this how to together was to combine all of the wonderful ideas and work into a single document. My comments are in this blue font.

The simple install places the heater switches on the back wall by the waterfall under the cup holder. This installation requires the least amount of disassembly and does not require any soldering. To place the switches on the dash, the switch harness needs to be extended about 40" and the entire dash needs to be disassembled.

The original instructions can be found at the following URL:

http://kappaperformance.com/index.php?topic=3772.new#new

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Remove seats. Each seat is held in by 4 bolts, 15mm.  The driver's seat has a power connector underneath and the passenger has the airbag sensor connector under it.  The seat belt is held on with a #50 torx. 

The following pictures provided by JPM from the saggy seat fix HowTo.

Remove the two 15 MM bolts on the front of the seat:

Slide the seat forward and remove the two bolts on the back of the seat.

Unplug the wiring harness from the seat:

As a precaution, I would remove the door sill before tilting the seat forward. The Seat belt bolt sticks out a bit and it can easily get caught on the corner of the door sill.

Tilt the seat to the side and remove the #50 Torx bolt on the seat belt. Disconnect the connectors on the cables to the seat.

Now you can remove the seats from the car.

Seat backs:
The back was the easiest.  Unzip both zippers to the top, then pull the flap to the side until it detaches from the bottom clip.  Pull back the side that doesn't have the release knob.  When separating the Velcro strips, keep a finger pressed against the Velcro on the seat to prevent it from separating.  Pull the cover up and over the top.  Install the heated seat kit as below in the photos.  Pull the cover back over and work the Velcro back into the groves as you lower the cover.  Tuck the heated seat cable to the side of the seat that is closest to the center of the car.   When you are ready to zip tie the cable below the seat, make sure the seat is full released forward and then secure the cable.  (if not, you will damage the cable the next time you move the seat back forward)

Seat Bottoms:
If you have seat adjuster - Pull switch bezel forward

Pull out switch

And disconnect connector

The Passenger seats have two plastic retainer clips. Pull these out using a door panel pry bar.

Passenger seat side piece held in with plug (Plastic Retainer clip)

The seat covers on the Solstice are held in place with Velcro strips. The heating pads on the seat bottom will need to have slits cut into them to expose the Velcro and allow the seat covers to re-attach to the Velcro. These pads have carbon fiber circuits that allow the middle of the pad to be cut as long as the circuits on the edge of the pad remain uncut. Make sure to avoid getting too close to the edges of the pad.

Place pad on seat and draw where to cut the pad over the seat Velcro

Place the heating pad on a hard surface

Use a ruler to mark the edge as to where to stop cutting; mark both sides

Use ruler as straight edge

Cut carefully on both sides of marked line. You can use a sharp knife or a pair of scissors.

Cut out section, I left about a half inch to the side element

Done cutting

Test fit on car and route cable

Peel off adhesive layer.  (Some sets don't come with and adhesive backing so you will need to spray glue on). note that the back is the side with the thicker material. The front has a thinner material and you can better see the outline of the heating elements.) Spray the glue on the back of the heating pads and on the seats. Wait a minute or so to allow the glue to "tack".

Lay the heating pad on the seat back and route the wires down the same side as the seat belt.

Zip tie on relay

Next two are passenger seat photos

Relay on passenger seat bottom

Now you need to decide where to mount the switches. Ideally they should be placed in a location easily accessible to the passenger and the driver.

This picture was provided by ELFF. He located the switches along the rear waterfall right by the Emergency brake panel. The advantage of this location is that both buttons are equally accessible to both passengers but they are difficult to reach. This is by far the easiest location to use. Simply drill a hole and put the switches in. in retrospect, I wish I had chosen this location when I did my switches. It requires the least amount of disassembly.

Remove the Emergency brake trim panel by prying up the rear of the panel and tilting it forward. Make sure the emergency brake lever is all the way up.

If you chose this location for your switches, drill two 3/4" holes in the locations shown and insert switches and leads, skip to the instructions for "seat installation ground and power" connections below.

If you chose to go with the dash mounted location skip to the dash installation instructions below.

Dash mounted switch installation

If you decided to place the switches on the dash, you have a lot more work ahead of you. Before installing the seats, cut the switch harness and add a 4 conductor trailer connector to the part of the harness that is on the seat.

Slide some heat shrink tubing onto each individual wire and then a larger piece over all 4 wires and the harness. Chances are the colors on the plug wires are not going to match the ones on the harness, just match them up as best as you can and keep track of which ones you connected together. The male connector (the one with the most exposed contacts) goes on the seat.

Solder the connections together, then slide the heat shrink tubing over each connections and shrink them with a heat gun. When you are finished pull the larger heat shrink tubing over all 4 connections and down to the plug. Shrink this tubing with a heat gun.

Solder a 40" of 16 gauge 4 conductor cable onto the other half of the harness with the mating switch connector.

Now you heed to disassemble the dash. Pry the cover off on the trim panel to the grab bar.

Remove the shifter trim ring. This is a two step operation. First pop the trim ring up. It will remain attached to the shifter boot. Once it is unseated, you need to remove it from the black plastic ring to which the shift boot is glued. Be careful with this, there are a lot of small tabs on the trim ring and they tend to break easily.

Remove the 10MM bolts on the grab bar. Note that you will not be able to remove the grab bar at this pointy but you need to loosen it up.

Now it's time to remove the boomerang. It’s easier to start at the lower left of the gauge cluster, and work around clockwise. Pull the boomerang straight out with your hands. Be careful near sides of the radio. There are two clips on each side and the boomerang is very thin there. carefully unplug all of the connectors starting with the cigarette lighter and working your way up. This is a difficult job and you need to press the connector release tabs which are located all over the place.

removed center console lower section. There are four 7mm bolts holding it in place. Then, you will need to disengage the tabs on the passenger side and pull it up.

Drilled two holes with step drill bit. The hole should be 3/4".

Routed wires beneath radio

The author mounted both switches together but it might be more convenient for your passenger if you mount the passenger side switch closer to the passenger side.

An alternate location is on the dash below the AC controls and above the radio as shown in TomatoSoup's car.

Although the switches appear to be quite low, the controls for the AC prevent them from being mounted any higher without modifying the AC control housing.

If you want to mount them a little higher, here are some pictures from 2KWK4U illustrating the modifications required and a final picture.

So, to do this, start by removing the boomerang and removing the AC controls which are attached to the boomerang with four 7MM bolts.

Now, Lay some masking tape parallel to the radio opening. Mark two lines on the tape parallel to the vertical edges of the radio opening.

Now mark a spot for a pilot hole dimensioned as shown.

Drill two 3/4" holes over through the pilot holes.

If you have a steady hand, you can use a 3/4" drill bit and drill straight through the AC control housing just barely gouging the bottom of the control panel. Otherwise, use a round file and gouge out a trench for the wires as shown in 2WK4U's pictures above.

Now, Pull the extended switch leads down through the dash opening alongside the AC/Heater cables. Keep the left switch lead to the left and the right lead to the right. Use a coat hanger or fish tape to route the extended switch leads next to the wiring harness by the E brake and down underneath the carpeting to the same spot where the seat cables are coming out. Make sure that all cables are routed behind the rear seat studs closest to the tunnel. Failure to do so will cause seat track to pinch the wires.

Once the switch lead has been pulled completely to the rug opening and all carpeting has been adjusted to lay flat, you can solder the female trailer connector to the switch lead using the procedure described above.

Seat installation, ground and power connections

Now you are ready to install the seats. Pull the white and red power cables under the rug and up to wiring harness by the ebrake. if you chose the rear wall location to mount he switches, pull the switch leads along with the power leads and plug the switch connector to the switch lead. Use a coat hanger as a fish tape to pull the wires.

Reconnect the connectors on the seat. Re-install and tighten the four 15MM seat bolts. Reattach the seat belt using the 50MM Torx bolt. Re-install the door sill plates.

Cut the "red" power and "white" ground wires to an appropriate size and splice them together. there is a ground stud by the ebrake handle. Strip the two white wires and insert them into an electrical eye lug. Slide some heat shrink tubing over the white wires first and then crimp the eyelet securely. Pull the heat shrink tubing over the lug and shrink it with a heat gun. Remove the 10mm nut from the grounding stud and insert the white wires onto the stud. Replace the nut and tighten it.

Pull the red wire under the rug and route it to the fuse panel in the passenger foot well. You will need to extend the red wire. The "red" wire needs to be connected to a switched power source. You can use a mini add a circuit on Fuse 21 of the fuse panel by the passenger foot well. the seat heater includes insert a 20 AMP fuse into the add a circuit to provide power to both seats.

Now reattach the ebrake trim panel and you are all done.

Finished product