1998 Lincoln Town Car ac stuck on deforst

Your problem sounds like you may have a vacuum leak. Under the hood towards the back of the engine near the passenger side fire wall, inspect the vacuum lines going in through the evaporator box. They are usually plastic lines that break and attaching a vacuum hose should take care of your problem. Just be care full when installing the hose between the plastic lines since they are very brittle it can break off more. Good luck and keep us posted please.

Checked for vacuum leaks,found no leak