The coils are not in the foam. That is a false internet rumor that has been floating around for years. One of our local Boxster owners has come up with a retrofit.
http://www.dennisvogel.com/boxster/heatedseats/
http://www.dennisvogel.com/boxster/heatedseats/
Here is a guy who tried years ago but gave up because the coils were not in the foam.
http://www.burnham.my-net-space.net/
http://www.burnham.my-net-space.net/