Here's a link to the instructions I used:
Dennis Vogel: Projects - Boxster Heated Seats
My favorite line in the detailed instructions is in the wiring section at the point where it is time to test your wiring. Dennis wrote, "Listen, look, and smell. If things seem out of sorts (popping sounds, smoke or sparks, burning smell), turn off the ignition and disconnect the battery immediately."
In the 986/996's the seat heater switches control relays on the seat bottoms that in turn control the amount of heat the seats generate. There may be some differences in your seats because they came from a newer generation car.
You may also find this link useful:
https://www.boxa.net/forum/topic/84441-986-heated-seat-retrofit-how-to-video/?tab=comments#comment-1171362 (You may need to register on Boxa.net - a U.K. Boxster site for this link to work.)
If your seats don't already have heating pads in them, I think you could use aftermarket seat heater pads and emulate the OEM control system.
I'll send you a PM with my contact information if you want to discuss it. Good luck.