1979 Lebaron heater/defroster issue

I've got a 79 chrysler lebaron s/w w/318 automatic and A/C (which needs recharging but its an R12 system which is a whole other issue).

The blower motor blows with the selector buttons (it's a vacuum system) in the a/c, max a/c, and vent positions.

However, when I select defroster or heat the blower motor stops blowing but I do hear something like a door moving around up inside the dash.

I live in Virginia where all vehicles are inspected annually, and since my defroster isn't blowing I can't pass inspection to get my sticker to drive this car legally!

The car was a freebie so I don't want to put too much into it if I don't have to. so far I've replaced the fuel pump, rebuilt the carb, replaced the muffler and tailpipe and done the brake cables. Other than this defroster issue it'll be good to go after it gets a good bath!

What should I start looking at, where should I start troubleshooting?

the door that you hear is what routes the air to the proper place, it may be moving too far to shut off air completely or you may have a plugged hose