As stated, the switch and the resistor coil pack are the 2 most common problems.
I would check the coil pack first....easier to get to/check. It should be obvious, if that is your problem....one or some of the coils will be burnt in two.
Most all the switches I have ran across have some heat damage to the connector....I guess the switch is not designed to handle the amp load....some will continue to work fine, some don't.