If you mention the year you have, we can fix you up in a jiffy.
Edit: Took it upon myself to get it together for whatever year.
92-95, Looking at the firewall of the engine bay there is a black plastic container next to the windshield wiper motor. This is the vacuum canister for the HVAC actuators. Ensure the vacuum line is plugged in and there are no cracks in the canister.
96-99, A bit more detailed and comes with a couple links.
Test: Pull the passengerside footwell hush panel and find the purple vacuum line. Test this line with the vehicle running for vacuum. If no vacuumis present, locate the vacuum canister under the passenger front fender and check from there to the engine and firewall. Find all vacuum leaks until you have vacuum inside at the purple hose.
If you have vacuum there is a long thread about this and how to test various components. Probably the most detailed thread about this type of problem ever.
The typical fix is documented at this link. Replacing small pieces of the vacuum hoses where connections were previously made corrects a collapsed nipple issue.