I had that exact problem on 2001 F150 4x4. I have a haynes manual so that helped me with the schematics and wiring.

What i did first was to check to see if the switch was getting any current and voltage at all. I did that and it registered 12.3 to 12.1 volts whish was good. An indication that volts were present at the switch.

The next step was to bypass the switch and manually cross the wires to activate the motor to see if it would come alive. It sure did come alive. So then i knew then it was a switch issue.

What i did was i took apart the switch cleaned out the contacts and it worked again like new. Hope this works for you sir....

all the best...