There's a few threads on the go about failure of the heated seats.
I'm starting a new one in what I perceive as the correct forum area.
Just over a year ago my driver's heated seat bottom failed - this was due to the first element, which is just at the back of your knees, this is where the power enters the seat cushion elements and is fead to the other 3 element (rear and each side). It failed again about a month ago, this time I have been told that I require a new seat cushion - they cost ~$400-$700 depending on VIN #. Originally I was planning on bypassing whichever element was broken - I assumed they were wired in series as none of the elements are warming up. After mulling it over I decided on installing one larger element, and just tap into the existing wiring.
There's two sites I've found which offer heating elements:
www.heatmyseats.com (my preference)
www.heatedseatkits.com

I plan on doing this over Christmas when I have some spare time - I'll let you know how it goes.
Attached are the installation instruction if anyone is interested.