Have you tried manually directing air to the defrost vents / dash vents / floor vents. If so, what were the results? If not, do that and report back what happened. That will help to isolate the problem.

 

That's what it sounds like to me. First question: Do you have JTIS or Alldata? That would be a must. My first thing would be to dig into it and find the blower box and clean it out and check all the ducting and dampers. By doing that you will eliminate or fix the cheap and easy first. I've had old cars get stuff in the blower box that would clog airflow, also had vacuum lines come off and duct hoses come off as well. Some would of thought it was a big $$ problem but once things were cleaned out and put back it worked just fine.... But I'm talking about old J cars and Monte Carlos....The cars of my youth...