An ascMcLaren forum for 84-86 Capri and 87-90 Mustang based cars modified by Automotive Speciality Corporation, a Division of American Sunroof Corporation. (no affiliation)
First I would like to give some tech/safety advice and let everyone know to inspect their heater resistor for corrosion/melted pigtail connection. I noticed my heater/vent fan was only working on HIGH. I pulled out the glovebox down and pulled out my resistor and it had minor corrosion. I changed it out and now all speeds work. (NOTE: One of the clips on the pigtail had been broken off in the past and a cheap resistor had been installed at one point. I replaced it with a Motorcraft unit. I then inspected my girlfriend's 92 5.0 and her resistor was is worse shape. The pigtail had signs of overheating and the resister was extremely rusted. Strangely enough her heater was still working on all speeds/settings.
Another tip for any 87-93(Not sure about 79-86) foxbody mustangs is when air only comes out of the defroster vents in any setting heat/vent/Ac. If this happens open the engine compartment, there is a Large Vacuum Tree on the LH side of the firewall. The line that goes to (AC) usually rots out and the vacuum leak will cause the default setting of the defroster to work only.
I Added a picture below, and Yes I will be changing the complete hardline out, finding aftermarket hard line is a little tricky.
