My 96' 2500 Ram does the same thing. It has done it for years. I have just dealt with it. I guess, I'll look at the vac line going into the cab as suggested. I saw a TSB before that had a diagram of which vac line needed an additional vac valve installed o cure the problem. I tried that as the diagram showed but then it stayed on defrost 100% of the time no matter where the switch was. So, I removed it.