I doubt that the switch would be the problem. But then there is a first time for everything. There is a much greater chance that the R-12 has leaked out. When you by-pass the switch look at the sight glass on the dryer. If the system has anything in it, the glass will look white as the freon begins to circulate. If it remains mostly clear there isn't enough refrigerant left to keep the switch closed.
Regards,
Randy D.