Why is wiper switch HOT?

Are the wipers on or been on ? High resistance from motor and linkage will heat the switch. The switch has a bimetal strip in it that will disconnect if it gets hot enough. This is meant to pretect the motor and switch when wipers are frozen to the glass. That bimetal strip will crack in half eventually. Its anyones guess as to how many heat / cool cycles before it cracks.
Also make sure the switch and the motor are well grounded.