I replaced the front heater pad on the drivers bottom seat this afternoon - there's 4 heating pads: front, rear and left & right bolster.
All the power comes through the front pad, and is then dispersed to the other 3 pads. The rear/back of the seat is on a different circuit. I haven't had time to try the heated seat to see if what I did fixed it or not - there didn't seem to be anything wrong with the part I replaced - but while I had it apart I though I may as well replace it. Took about 2-3 hours to do the whole thing - if I had to do it again I think I could do it in 2 hours. It's really simple to take the seat cushion and cover off - and a lot easier to put it back on.
I took a few pics while I was doing it, so I'll do a quick write-up which can be used in conjunction with the instructions I've already posted.