Hello all, this is meant to be a tutorial for installing aftermarket seat heaters. Hopefully it will be helpful...
First of all thanks to Dick B and to Huskieowner for their valuable advice.
The kits I installed are H3 kits from heatedseatkits.com (with whom I have no affiliation). Other vendors sell very similar products. I was able to get the kits for $65 direct from them when I ordered 4 or more kits.
The first thing i did was to remove the seats :
on the outside seat rail in the front, there is a plastic cover that hides one of the four star pattern bolts. I removed it by prying gently with a small screwdriver (for this project, they should be called pry-ers since they have no other purpose)
slide it forward and it should come off... revealing one of the four bolts.
These bolts can be removed with a 10mm wrench. Two hints : 1) these bolts were tight and I could not budge a couple of them - so I used a length of pipe to insert over the wrench end to give my lever a bit more length. 2) adjusting the height, angle of recline and the fore/aft position will help expose the bolts.
There are two small wiring harnesses attatched to the seat bottom - remove these
Hint: place a thick beach towel on the door sill to prevent scratching - really
After the bolts are undone, you can now tilt up the seat away from you to facilitate removing the seat belt bolt. Pry up the plastic cover and remove the bolt with a 17mm wrench.
Hint: when replacing this bolt when reinstalling the seats, make sure that the belt is not twisted.
I placed the seat into the back of my 4Runner to give me a work space easier on my back (Thank you Huskieowner!) Although i dont have any pictures of this, on the back of the seat, you can pry up the material where it meets the plastic shell with a small screwdriver. I could not figure out how to get the headrest tunnels out - and it proves unnecessary. At the bottom of the seat in the back - there is a strip of cardboard that is inserted into a retaining slot - pry this up. By this time you've noticed that the seat back material is held in place with small metal clips with treachorously sharp points.
Hint: adjusting the angle of recline proves helpful...
Down the two sides of the seatback and the center (in my seats at least) are three metal rods that are held down with plastic clips. These rods help from the channels of the seatback.
On all three rods, I had to remove two out of the tree clips (starting at the bottom) to give me enough working space. The heating element will actually be pulled down into the center channel by the rod. So once the rods were disconnected and I could lay the heating element onto the foam and position it where i wanted it, on the heating element I used a marker to mark the spots where the clips had to go through. I cut the fabric of the element without cutting the carbon fiber honeycomb
Now this is important! Once you reinstall middle rod, it pulls the element into the center channel - take your time and smooth out the element as best you can - taping the element in place might me considered. Once you have taken the rods out of the clips and replaced them, you will not be in the mood to do this again to smooth the element out! really..... i was soooo mad....
Reclip the two outside rods. Making sure that the wiring harness is going to end up where you want it - toward the center of the car, replace the fabric. I wish I had some hints about this - it is not easy..... but a million times easier than... the bottom cushion!
I just started prying out the three cardboard strips around the bottom perimeter
There are plastic clips that hold on the three metal rods as in the seatback. I just cut these off. I just didnt have the patience to work each rod off the clip... plus these looked way harder to reinstall. As in the seatback, place the heating element (this element is supposed to be the one with 4 wires in the harness). I bore a hole through the foam for the wiring harness and actually made a small channel for the bump in the healting element - a sensor I think). I replaced the metal rods by using military grade zip ties ( I got a shrug from the hardware store when I asked what 'military grade' means when it comes to zip ties).
Replacing the fabric on the bottom cushion is not easy. I spent a lot of time trying to place the cardboard strips back into their retaining slots - I cannot fathom an easy way even in retrospect. Just time and patience! Putting some weight on the seat ( I was lying on it) seemed to maybe help.
I placed to switches on the flat plate under the shifter - right beneath the window controls.