Mindframe, the seat heater elements are woven into a fabric liner of the seat. So, you have 2 options. I will discuss this later. As for what your problem is, odds are it is the heating elements. Keep in mind that you have 2 of them. 1 is in the seat bottom, the other is located in the lower back of the seat back. To access them you have to pull the skin off of the seat which will then expose a white sleeve that is over the foam rubber. This white sleeve houses the heating element. Odds are, when you get access to this white sleeve, you will see some black/dark brown spots in it. That is where your heating elements most likely failed.
Now, for the repair. The obvious one is putting new white sleeves in. But, stand by for what they cost. I think you might consider option 2 first.
The other repair is to use a multimeter and probe to either side of the burn marks till you can find good heater element. This is going to involve ripping the white fabric to expose the metal banding that is underneath. Once you have good metal, you will need to cut a piece of wire that is about 1" longer than the space between the two points of good metal. Strip back about 1/2" of wire on each end of the wire and then solder the wire to bridge over the burn spot. Do this for each and every dark spot. As for wiring, I would say to use something like 16 gauge or 14 gauge stranded wire. The key being stranded wire. You need something that is flexible so it won't break on you in the near future.
