[Honest Jim]

Kap,
Sounds like the same problem I had recently. Here in Northern Ohio last month My 94 Bonneville out of the blue just started blowing cold air. I read all I could, checked wires vacuums, etc. to no avail. I noticed that if I ran the thermo setting down to 60 and let the a/c kick in, then run it back up to about 75 it would work, for awhile. Also, that if I popped trannie in neutral and shut the engine off and restart, it would be fine for awhile. (Though, I don't know if that is any good on either the engine or trans) but in sub-zero temps you do what you have too. I finally broke down and took it to a mechanic.
He was able to wire the "door" or flap open for heat to enter the cabin, and all settings work fine. Though I believe the flap actuator?? which is in the dash needs replaced. He said it is a job for you have to tear the dash apart to get to it, and would run about 400-500 $. I did not delve further into it with him at the time, as it was late night and it took him all day to track down the problem. I will look for my receipt in order to give you an idea what he did do if you like. But, he did say he had to reprogram something, I don't recall what it was. I was happy to just have constant heat. Hope this helps you. Jim