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I just replaced mine about a month ago, bought it at a local auto parts store, rather pricey and I could only get the one that goes to the drivers side vent. I was able to repair the one that went to the center dash vents
I could only get the one that goes to the drivers side vent.
When I was rounding up parts to swap a/c into my '73, I found out that '79s have solid plastic ducting going to the driver's vent. One less part to get holes in...
I have a 1978 F150 2wd, a 1978 F150 4wd, and a 1978 Bronco. All are 78' models, all are Ranger models specifically and all have factory A/C. Only the 2wd has the solid ductwork. The other (2) have flexible ductwork. The (2) vehicles with flexible ductwork are full of holes, so the heat sucks. The solid ductwork is solid, and that's about what I am about to rob from the 2wd to throw into the 4wd that I drive all the time. I am sure you can come up with something to work, but wouldn't a junkyard have plenty?
For the flexible kind, I'd think you could walk around a junkyard and just look inside most anything - Ford section, GM section, Mopar section, etc. Pretty common stuff for it's era, so I'd just have a walk and look into cars and see what might have already been pulled in the dash to open the area up for you.