I have a 98 Dodge Dakota with about 100,000. While operating the heating or air conditioning, the blower doors periodically close all by themselves stopping any air circulation through the upper or lower vents or the windshield defogger. If I turn it off for a little while and turn it back on, all is fine again, for a while.
My mechanic has had the dash completely apart to replace the heater core and has found nothing wrong with the opening or closing of the doors. He has also checked the vacuum hoses and found nothing wrong.
I don’t know what else to do. Needless to say, it can be a bit hazardous when you need the windshield defogger and it doesn’t work.
Your mechanic probably messed things up when he replaced the heater core…It’s all plastic junk under there and it’s almost impossible to perform this job and have everything work correctly after reassembly…