My brother-in-law just bought a 92 Bonneville and he can only get air through the defrosters. He says he has vacuum at the ports under the dash. I suspect a control door is stuck - any suggestions?

Billboost will testify to my favorite suggestion:

Look for a vacuum line going into the firewall to the left of the brake master cylinder. There should be a black box there with a vacuum line running from it to the TB area. That one supplied vacuum to the HVAC system.

had the same problem and the vacume line was off right at the black box in the engine bay.

Great - thanks - I will have him check out your suggestions...

If you have vacuum confirmed inside then there is a good chance there is a bad vacuum actuator. Although it is fairly uncommon.

Missed that part. Could this be the valve behind the glove box?

There are 3-5 valves which I've never gotten into. Unfortunately I don't think most of them can be accessed w/o pulling the dash. Although..the dash is an easy thing to pull.