Freking int. wipers,lol! Never really saw much problems with customers, BUT, generally, older vehicles never came to the dealerships or quality(expensive) shops I worked at. So, much of my experience is from my own K20, which is a dedicated plow truck, sits all summer up here, chained up, never goes to town. Instead of really long different stories, will just give experiences, thoughts and what I have found over the years-
The ground wire/system for the wiper motor. It was down on the left kick panel(well above a bit) inside the truck. Known for bad connections. Will cause all the problems listed. Location may be different in the 80's??
Dirty rheostat in the switch for intermittent. New switches are cheap.
Yes, park switches do wear out, can be replaced without replacing the motor.
Motors get weak, especially as the brushes wear down, lose tension.
Poor quality rebuilt motors. Had this happen to me.
Now 2 of the most important--
Wiper linkage. 2 things happen, the bushings bet worn out. The grease gets old/hard. If you suspect either, pull off the arms, see if the shafts wobble. Even if they do not, now unhook the motor, see if it feels like too much resistance, even if bushings are good. Push against a wiper transmission shaft while testing, see if it "drags". Now this is an easy fix, for most, even if linkage is shot, but your truck is occasional use--2 things replace, as replacements are available, OR, do As I have done, along with many others-pull linkage, drill a small hole in each pivot, about in the middle. Use a needle point on your grease gun, pump until all the old,hard grease comes out, and clean comes out. Over the years many have reported even on a "good" wiper system this speeds up the wipers a lot. Yes, I can verify this. Yes, the dragging/resistance can cause all the above problems, including burning out the module, or switch.
Now something a bit different--low voltage. Yes, it also can cause the problems listed, it "whigs out" the module. I still experience this on my plow truck-with greased linkage, newer motor, new switch, 2 known good modules, 105A alternator, when I am plowing, headlights, heater,wipers on, and lifting/angling plow a lot in a short period of time. BUT, I run a very heavy plow, draws lots of amps. Very annoying, but is nothing else I can do short of a bigger alternator. Yes, this is with 2 batteries,lol. The neighbors K's I have just put regular wiper switches in, took out the INT., as they were tired of it,lol.
Hope this can help some.
