My 2005 GXP seat heater stopped working. It turned out to be a broken wire accessable from under the seat. That apparently also burned out the module; also under the seat. I switched modules with the passenger seat. Module was about $75? from GMDirect. I haven't replaced it yet.
Go here for some pictures and the rest of the story:
https://www.gmforum.com/bonneville-gxp-northstar-powered-cars-107/another-seat-heater-module-story-281121/

cannot get heated seat fix site???