Ford Repair: defroster, clogged drain, c case

Question
When I turn on the defroster on my 89 F 150, it causes the windshiled to fog over.  Any ideas?  Thanks, Ricky

Answer
Hello Ricky, The fogging may not necessary mean there is a issue but, Check for a possible heater core leaking or a clogged drain for the heater a/c case. When fogging occurs there is moisture or humidity and temp. change between inside and outside the vehicle in the case. Hope this helps Thanks Tony