1992 chev. 4X4 c/k 1500 heater/AC motor does not run on high, but works well on low and med.

Okay where is the evaporator case at?

I know the answer to this because I had the same problem. Open the glove box. Take the insert out that is held in with the four screws. By insert I mean the tray everything in the glove box sets in. Once that is out you will see 3 or 4 (should be 4) relays (little square box looking things) all clipped to a bracket which is held in by two screws. Each relay is plugged in to its own connector. Now because I don't know which of the four control which fan speed I will say you need to replace all 4. No worries. It's easy to find those at any junk yard. Find the ones in the junkyard that have the four relays side by side just identical to what you have in your truck. I got all four of mine for 6 dollars. Now all fan speeds work. You can't miss it after you get the tray for the glove box pulled out. The four relays will be slightly to the left and center.