Hi mate,
From what I can see having looked at diagrams as much as I could, the heater elements are built into the backrest cover, and the base cover.
Whether that means the thermistor is part of those is another question, and if so, are there 2? As then it's a case of buying both covers to save time, and I suspect £££ for those!
I might just sell the seats to be honest! And buy a full leather interior! If VW are going to charge me £3-500 it's not worth me bothering to fix it, i'll just sell the seats to someone who doesn't care about the heating part! (And make sure they are 2007+ seats I buy!)