I have a 2002 expo i have no heat in front but i have slight heat in back i have replaced thermostat, flushed heater core and still no heat so i figured blend door is open so instead of tearing out dash i cut the heater box cause the truck is old and ready for junk yard i cut the hole and closed the blend door figuring that would give me heat but no heat it almost feels like the ac is on why wouldnt i get any heat by closing the blend door