The blower works in all 4 switch positions, but position 3 is the same speed as position 2. Otherwise, positions 1, 2, and 4 run at the proper speeds. I replaced the blower resistor, but it does not seem to be the problem.

Any ideas for where to look next?

 

Eh, never mind. Judging by the partially-melted connector and other obvious signs, looks like I need a new blower speed selector.