Good morning, I had the same issue, prior to pulling the controls (PIA), crawl under drivers side, remove the duct that crosses under the steering wheel area (two phillips screws). Take a flashlight and look toward the center of the vehicle, there is a white pivot assy that controls the air flow, while moving the cable to defrost, watch the white pivot assy, it should move to where there is a notch in the assy. Mine wouldn't go past this notch (to defrost), I had to clean years of goop off the pivot assy and lube, while lubricating I continued to slide the selection lever back and fourth. After about ten minutes, it would going into each selection position with out issue.

I was going to pull the cable selection assy from the dash to review until I looked under the dash from the drivers side. Easy to clean and this might resolve issue, if the cable is broken, it is easy to change without removing the lever assy from dash (this one cable). There is a salvage yard in Springfield MO (Buddys U-Pull) that could have the cable, I have been buying several parts for my 1990 LWB from them and so far they have had all items requested.