To test if the diode is bad, disconnect the headlight motor connector. Take a multimeter and set it to resistance testing, and place the black lead on the black wire, and the red lead on either the grey/black wire or the green/black (I think). This is off the top of my head, so you might have to futz with it. Anyway, you should end up with conductivity in one direction across those leads, and none in the other. I’m going to assume that the diode is bad.
To be honest, it’s best to do the following procedure to both headlight motors, because the diodes will eventually go bad. First, disconnect the battery. A lot of procedures that require disconnecting the battery don’t really need it all that much, but this one does simply due to the annoyance factor of the motor jumping around on you when the engine is off.
Anyway, take the headlight motors out. Then, clean them really well with purple cleaner or some other good solvent. Partially this is to make it more pleasant to work with, and this is also to make it easier to get to the diodes.
On the ’81s the diodes are underneath some icky plastic coating which you will have to chip away. Once it’s gone you’ll have access to the diodes which are located inside of heat-shrink tubing. Cut away the tubing, and the odds are the bad diode will be split in half.
Now, the replacement diode is up for debate, and I’m sure someone can recommend the best substitution. But, I have a ton of diodes laying around, and usually just grab whatever large 12V one I can findit’s worked well so far. So when you go to RadioShack just go to the diode section and find the largest black 12V diode you can (you don’t want the clear glass ones). Take a short length of appropriately sized heat-shrink tubing and place it on the wire. Then, solder in the diode with the arrow pointing toward the motor (or the white line on the motor side).
Hook up the connectors and place the headlamp motors on the fenders on top of some towels (to keep them from scratching the paint). Reconnect your battery lead and probably one of the motors will start to spin. Doesn’t matter. Then, turn the headlights on and off and they will whiz around.