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working a/c? modern cars the a/c kicks on whenever you have on the defrosters (at least gm)
Are you sure your heater box is up to snuff? I have rebuilt 3 of these now and this is what I can tell you from my experience. The main problem with defrost in these trucks is the foam on the end of the defrost door! It gets old and brittle and falls away!I have used pliers and bent open the staples that hold the foam on, removed the remnants of the old foam, used some wide foam like that used to seal canopies to the bedsides (about 1-1/4" wide, 3/16" thick with a PSA backing...going off of memory here, but just that roll of pickup and camper tape that is wide enough to cover the defrost door) and put the staples back in and bent them back over.Make sure your blend and mode doors are functioning properly, put new foam tape around the perimeter of the heater core opening and couple all of that with a new blower motor...maybe even a new heater core while you have the thing apart and you will be amazed how well it works!