Thanks to all who have tried to help with this chilly problem.
First, I know there is no leak from the heater core, as this actually happened a couple of years ago, leading to the core's replacement. When I'd use the switches (yes, they're cable-operated), I could hear the little doors in the plenum open & close, so I know they were working ok. Unfortunately, I decided to double-check the operation for myself yesterday & once I got my fingers inside the plenum the whole thing literally fell apart! (why in the world did Ford use cardboard?) Talked to my mechanic & told him one of you nice people had suggested a vacuum problem, which he decided could very well be what's going on. Since the plenum is shot, he's going to go thru the system from radiator to switches, replace the necessary parts, and cross his fingers. Hopefully, I'll have heat before summer! Libby