Check these guys out.
http://www.heatedseatkits.com/faq.html
I bought an 02 XLE this last spring with 68,000 miles. Naturally, the driver side heated seat element was broken. I had three options, find the fracture in the wire in the original heater and repair it, buy a new OEM heater, or put in an aftermarket heater.
I opted not to repair because the fracture in the element was at the base of the seat where the heating element gets the most wear from the driver getting in and out of the seat. I was afraid that the repair would be temporary.
OEM element was not an option as the best price I found was $460. The dealer I called wanted $650! He said "They must not go out that often as we don't have any in stock and once a part is ordered 3X, we automatically keep it in inventory." I believe they don't stock it because the heater rarely fails within warranty and no sane person is going to pay the dealer $650 plus labor to fix a seat heater. And I would have had the same element design that failed once already.
I ended up getting the aftermarket heater for $60, removed the seat from the vehicle, took off the leather covering and replaced it. The new elements are in a "honeycomb" design rather than one continuous wire. Even if a portion of the element was to fracture, it would still have continuity. A multimeter that tests voltage and continuity is a handy device to have when troubleshooting.
The installation was pretty easy and I'm looking at adding a pair of heaters to my 97 Tahoe.