A/C blower motor ??

i was having the same problem with mine and behind and rite under the glove box tucked in toward u ther r a couple of wired. just lik mine one of those transistors r fried or one of the wires is not getting a good ground.

I had this issue on my '88 B2200, had all speeds but almost no volume of air came out the vents, turned out there was a lot of gunk built up on the AC evaporator from a leak. Some refrigeration oil mixed with dust, causing blockage. I replaced the evaporator to repair.