Look for the the small black wire that goes from the seat bottom to the seat top (at the pivot point). There is a pad in the seat bottom and in the seat back. Hard to find them and if missing, you will have to fish for them under the upholstery. If you have a broken wire there, the seat will not heat. Check at the wires under the seat for continuity. The black wires are the heating element wires. A lack of continuity will be the clue that the previously mentioned wire is broken. I think the switches are pretty good on these and wouldn't be my first suspect. My old L seat stopped working a few years prior and when I pulled them to install the S seats, I discovered the broken wire in the old seat. I had known about the lack of continuity, but didn't know where the problem was until discovering that. I used mine this morning and it was about 50*F this morning, so it doesn't have to get that cold for them to work.
Charles