most likely it is a connector or a faulty ground, if one lights up, like he said, then your switch should be OK,(it doesnt make sense to have 2 switches) .. actually what i did on an old, crappy previous car, well i had a short in the passenger side parking lights(the fuse kept burning out, and the rear-right tail light and front right corner light would nto be illuminated) well when i got pulled over and was ordered to fix it in order to have my ticket dismissed i tapped a wire from the passenger side to the driver side and it worked like a charm
and as for the heated seats(take it from someone who actually took their seat apart, yes the cushion) it is a tray of wires that goes between the cushion and the leather, its really a pain if it was actually the seat heater, so lets hope its the switch