Hello. I have a 1998 GS300 that was originally ordered in San Francisco and predictably did not get optioned with heated seats. There is no definitive answers when it comes to adding them after the fact, so here is what I learned adding them to my car.

You will need
* OEM heated seats. They are different than non-heated and do not have the heating elements.
* Center console fascia with heated seat buttons between the transmission PWR/SNOW switch and VSC OFF button.

That's it. You will need to remove your center console assembly by pulling the ashtray/cigarette lighter out and unscrewing two JIS screws hidden in there, unscrewing your shift ****, and then prying up the trim piece. Pry it from the rearward edge where it meets the black center console cubby. Please be careful - fragile plastic abounds. Lift up the center console fascia and remove the two plugs that will be in a non-heated car for the VSC and PWR switches. Remove the assembly. It's finicky.

Luckily for you, Toyota placed the connector for heated seats even in cars without it and it is completely wired! There is a dummy plug in it from the factory, and if you are following an EWD, the connector is S3. Remove the dummy plug and reinstall your center console fascia that has the heated seat switches. Check the passenger foot well fuse box and make sure the SEAT-HTR fuse is present; on my car, it was already installed. Next is to remove the seats. Slide them all the way forward, remove the trim pieces covering the two rear bolts, and then undo the bolts. Slide the seats back, and undo the front bolts. Then disconnect the battery and wait 90 seconds to disarm the airbag system.

Under the seats are two connectors: an airbag connector, and connector BC1. There is no wiring to do; BC1 already has the terminal for heated seats no matter what. Simply remove the two connectors, remove the seat, and place in your new seats. Connect wiring, bolt down, and you're done. That's it.

The seats between heated and non-heated are different in wiring and to a certain extent upholstery. Heated seats will have the heating elements in a fabric sheet which is sewn into the structure of the seat. It will also have a few two-terminal connectors between heating elements and the BC1 connector, with terminals and junctions to match. If you have really nice OEM non-heated seats, it is possible to open up the seat and add a two-terminal heating pad, either aftermarket or from a ratty OEM heated seat. If you are salvaging from an OEM seat, it will not be pretty. You will have to rip the seams or cut the sheet holding the element. I did that for my car and I do not recommend it. You will then need to add a terminal to the BC1 connector and route it to your heating pads, and splice the other end to ground.

Also, OEM seat heaters kinda suck, even in factory cars. I've sat in a 2002 GS with heaters and I would more describe them "mildly warm". Better than freezing leather in NY winters, but not hot for sure.

See below for the OEM wiring diagram for seat heaters:

 

Ya the factory ones aren’t that good. I would get the factory switches and wiring but I would keep your seats and just buy an after market hearing element. They aren’t difficult to install they will work better than OEM, and u can make em like OEM. U just got some single mode with the upper upper and lower pads. They should last longer than OEM also.

 

Thanks for posting this. I was given a second center console fascia with the heated seat switches on it and was curious how to make it work in my 2001 GS300. If I weren’t getting rid of the car I’d find some other seats and set it up.