Blower switch problem
 

I have a 1975 K20 for a plow truck. The blower switch has gone bad, and I cannot get anything but low speed on the blower. Every replacement switch I have found has four terminals and will not fit the connector, as my switch has only three terminals. I tried to modify a replacement... didn't work. Anyone have an idea of where to get the proper replacement or a way to bypass or replace this switch with something else to get that blower working at speed? Kinda difficult to plow snow when the windshield won't defrost!
On this old rig the blower runs all the time, and the switch changes terminals on the resistor? Any assistance or good ideas to fix/change this system and get it working would be really appreciated!