Nine out of ten times, it's the heating element in the seat. They are available and require removal of the seat cover, top and back. There are some aftermarket elements available for less money. Corbeau lists one for their seats (do a Google for 'Corbeau Seats')
To determine if it is the element, find the plug underneath the rear of the seat, and check it for voltage. If It's there, it's almost certainly the element. OEM elements don't seem to last too long.