The Z3 ones though do seem to get very hot
Could be a sign that things are on their way out. Here's the latest broken wire, next to be the bit I soldered in last time
And the corresponding burn in the foam
I thought at the time that it was getting a bit hot, then it stopped heating at all. Guess there was a reason.

Not finished the job yet, still got the backrest part to do, and then a bit of soldering. The replacement mats have a two pin plug on them, shame it doesn't fit the BMW one. Tomorrow will do, don't need heated seats at the moment, but I'd rather do it while the sun shines and I can drop the roof down. Then I'll find out just how toasty the new bits are.