Finally a question I can answer! I have the same problem with my 90 745T. The problem is that a flat rubber washer/gasket thingy get in the way of the selector lever. If you push the selector hard enough, it will come out of defrost mode. Permanently.

The solution is simple: pop off the trim around the climate control levers, remove four phillips head screws and ease the controls out of the way, and using something long and thin push the flat gasket out of the way while moving the lever. It's almost easier to do than to describe.

Bob