on my 89 GL. Its stuck on heat,but I want to be able to switch to defrost,bilevel,etc....fan works fine. Is this a vacum operated deal? Where is the best place to start troubleshooting? thanks!

Vacuum hose hooked to the intake manifold has come loose somewhere - trace the hose and find the leak.

 

GD

Yes, the system is vacuum operated. Perhaps the place to start checking is in the engine compartment, right (passenger side) firewall, there should be a small milky-white plastic vacuum reservoir a little smaller than a soda can. These tend to leak as they age, but also the hoses could be off or damaged, or the one-way valve in the engine-side hose could be missing or backwards.