Look for a vacuum leak. Start at the connections on the vacuum canister between the windshield wiper motor and the maxifuse relay center. Black plastic. Underneath, you'll find the connections.
If you find no problems there, you need to pull the hushpanel from the right front and inspect all the vacuum lines coming from and going to the HVAC controller. All different colors. You'll see them.
The door that directs where the air goes is controlled by vacuum on the ECC-equipped cars.
EDIT. Just realized you're a 98. I think your vac canister is in one or the other front fender.