When I turn on my wipers, they just hum and do not work. on 1993 GMC 1500 Pickup

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Sometimes you can turn the wipers on high and move the wiper arm and then they may work. Also, when the wipers do work, they will stop in mid-wipe.
(2) Answers
Remove the wiper arms and cowel cover. Check the pivot points at wiper assembly. They may be rusted not allowing wipers to move properly.
I had that problem. There is a circuit board somewhere near or attached to the motor. It was flexing and causing problems according to temperature and humidity. I found the problem: a solder spot on the circuit board. I Fixed it and it came back. That board is a tight fit causing it to re-crack. Rather than buy one at a dealer, I soldered a piece of thermostat wire across the solder crack. Never had another problem in 10 years.