Mine does the same.
There's a lot of discussions (several, anyway) on fixing the problem, so give it a search.
I read some of them, a good while ago, and they sounded like more of a challenge than I wanted to undertake just to heat the seat. I always seem to find something else to work on that feels more important to me at the time. She hibernates in the winter months, so being a fair-weather Boxster driver, I just forgot about it. I've adapted...top down/windows up + heat cranked up, it's gotta get pretty cool before I even start to miss the butt-heater.
