Just had my M5 in for this problem, the diagnosed as the a drivers seat short, disconnected the seat and replaced the
fuse while waiting for the heating pad.
Went back in on 2001.02.26, sent it to the upholstery shop to have the seat opened up so they could replace the pad
only to find that it was the wrong part, figured out the correct part number for the sport seat, put the seat back
together, just picked it up today 2001.03.01.
So if you have the sport seats make shure the dealer knows that there are three pads (not one), also the part is on back order and the dealer (Allison in Mt. View) had know idea when it might be in.
I used the heated seat a few times on the drive back home from the dealer last year (Edison NJ to Sunyvale CA) then
went use the seat a year later and it was bad.