There was a lengthy heated seat thread in electrical section a while back, pre face lifts are simpler apparently due to the on off button, where as face lifts have a stat in the seat due to the multiple position switch to give adjustable heat levels and it seams the stat may be the issue in my two getting too hot.
The two the don't work are both face lift also and will be due to a broken wire in the seat cover. The base and back are wired in series apparently, so if a wire breaks the whole seat fails to heat. There is a small deformaty in the leather on the drivers seat so I guess that's where it failed.
The real bad news is they don't sell the covers any more, and the wiring is integral to the seat cover, woven in so to speak, unlike the BMW seat I repaired with a neighbour where there is a cotton pad with elements in under the cover instead of in it, which might be a way forward if we can get a suitable plug on the wires. Pads ain't cheap, and obviously involves removing the seat from the car and removing the cover, which is fiddly but doable.
Iirc LincsRobert used to repair the covers as a bit of a service. So wonder if he'd accept a seat cover for repair through the post?