intermittent a/c blower!!!

The blower for my trucks a/c acts up intermittenly, sometimes it works sometimes it does not. When it does go out it is out on all speeds, even high, and checking for 12volts at the blower motor it is not there when this situation happens. I think the motor is good. Could it be the fan speed switch???? or is there some other type of relay??? Can the fan speed switch be changed by itself or do you have to buy a whole new a/c control module with all 3 switches?????? How hard is this to install??

I know I have asked alot of questions but trying to find out the root cause of this is driving me crazy!!! LOL!

Any help or response is very much appreciated......Thank YOU!!!!