Hah you're trying to do almost exactly the same thing as me.

First step - double check fuses and relays. IIRC, there are 2 fuses - number 26 at 5A, and number 32 at 25 or 30A, and one relay (number 7? For some reason, that's stuck in my head - don't quote me on any of this) - just make sure they're all there and passing voltage properly.

If the above doesn't fix your issue, bring your X in and tell them to look up these part numbers in the HG52 (reference/repair manual)

01290390167 - Owners manual pages, heated seat retrofit (not 100% necessary)
01290392287 - Repair procedure, heated seat retrofit, LF seat
01290392288 - Repair procedure, heated seat retrofit, RF seat

The way it's been explained to me by my local stealership, the SZM (switch module you posted about in another thread) signals the ECM (Electrical Control Module), and the ECM may need reprogramming to retrofit the heated seat signaling.

HTH!
Erik