I've got an 89 420sel...question is how can you work the AC without having the windshied defrost vent come on? The windshield fogs up(condensation on outside) whenever AC is on and weather is humid.

Nick it probably isn't a problem with function. Two common problems (but not limited to) are a bad vacuum pod or a failed vacuum changeover valve that cause this condition. On the lighter side lack of power to the changeover valve can be suspect. If you are fortunate, cleaning the contacts can sometimes be the cure. Ultimately you are going to have to go in and take a look.
Don't get intimidated by what seems to be an engineering nightmare
This is a forum link that will give you some real good information from some real knowledgeable members
Climate Control:

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