If your car is still under warranty take it in and get the heating pad replaced. My 02 325iT had a VERY hot seat heater and was corrected by taking apart the seat and UNFOLDING the pad which had been installed incorrectly.
I had a 98 MB that had to have a seat heating pad replaced due to burnout.
Word to the wise--the seat heating wires are very thin. You can damage the wires simply by kneeling on the seat. The excess point pressure from your knee will stretch and snap the wires.
Own 03 325iT; MB 02 C230 Sport Coupe