Truck is an 81 and the right windshield wiper doesn't work..What causes this?

 

Check to see if you have power at the wiper motor plug. Does it have ground. Does the wiper motor have continuity thru it. If not look in the circuit, fuse, switch, bad plug, burnt wire, bad ground condition. If motor is toast replace it. A VOM or Circuit Tester from Lowes will do the tests. Run your own ground wire if needed. Rust at your body grounds may be the culprit. Are the brake lights, parking lights, horn acting up?

 

If it's just the right one, and the left one works, it's probably the linkage. You have to remove the cowl to access it.

 

At any rate LMC Trucks has all of the parts, see below.