1989 Jeep Cherokee no change in air flow

Your directional doors for hvac may be partly vacuum controled. If there is a vacuum line going through the firewall, I'd start checking there. There should be vacuum there with the engine running. If no vacuum at the hose backtrack to the vacuum source under the hood.

If all that checks out, you can take the hvac control loose from the dash, look for the cable/s connected to the levers, backtrack the cable. If you don't like working under the dash, and I don't, tracking this down can be a pain. Maybe you'll get lucky.