I have been around for 6 years and that 1998 message is still being repeated. I do not know anyone who has found the heaters - it is easy to tell because you can see the wires. I have taken the backs off several seats and there were no heaters. This guy could not find them either.
http://www.burnham.my-net-space.net/
The heater is part of the foam. Foam for the bottom and top. There is one part number if the foam is heated and another part number if it is not. No reason to have 2 part numbers if the foam is heated in all seats.
Notice that the guy said "he found out" the heating element is in all seats. No clue what that means. I found out it is not true by pulling off the seat backs on cars which takes a few seconds. No wires = no heat.
This guy came up with a retrofit. But he had to buy the foam that was heated.
http://www.dennisvogel.com/boxster/heatedseats/index.html