There's a vacuum hose routing diagram under the hood that will show that hose. Ford typically used a black can about the size of a large tomato juice can mounted on an inner fender for a vacuum storage tank. A vacuum hose goes from that can to a plastic tree on the firewall, and another hose goes through the firewall to the heater control switch.
Most systems are spring-loaded to go to defrost mode as a safety measure when there's a problem with the vacuum source. Your feet might freeze but your windshield will be clear.
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Saturday, December 27th, 2014 AT 4:01 PM