The windshield defroster works ok until I go up in elevation and the outside temp drops to around freezing.

THEN the windshield fogs up and I can't see the road. I wipe it off but it continues to fog. I have to pull over to avoid getting in an accident. After much wiping and waiting a few minutes for the defroster to warm up the window, I can continue but it will happen again and again until I go over the pass and the outside temp gets up above freezing. The defroster air blows out one vent in the CENTER fo the dashboard and only defrosts a small section of the windshield right aboved it. The rest of the windshield fogs up.