The 97 I am assuming is like my 98. And i'm assuming you have the automatic temperature control.
The easiest to check is to see if you have vacuum at the tank under the right front fender after the car is shut off. Pull off the connector and then run the motor and put the connector on for a while to see if you have vacuum there.
The surest way is to take down the hush panel above the passenger'* feet. Then look at the right side of the car for the black line coming from the engine compartment. It connects to the violet line going to the programmer box. Pull that apart.
If you have vacuum there with motor running then you've narrowed problem to the interior. There is a problem with the rubber nipples inside the rubbery plastic conntector in the corner of the programmer box collapsing. The one for the vacuum control the operates the vane that closes off the defrost channel and makes the air come out the dash vents is often the one to show the problem.
Check the various tubes where they go tot he 3 or 4 vacuum motors above the hump and above the driver'* foot.
If you find them all intact and you believe it'* the nipples collapsing rather than electrical solenoids not changing inside the programmer, take the plastic connector out of the programmer. I listed which color of tube inside the programmer connected to which color outside the programmer. Then I cut out the 7 tube connector and reconnected each of the lines with 1/8 inch gas hose. About 1.5 inches on each; little liquid and they slipped right into both ends of the gas line.
My car would switch to heat and heat and defrost mode. So I felt the controls were okay inside the programmer.
If you try to pull the soft plastic connector apart the nipples inside will come apart and you will need to replace it/bypass it anyhow.
I have a link for pictures for this but that is on another computer...