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Yep. Sometimes, the rheostat burns out or the wiper gets bent or dirty. If you pull the switch out of the dash, you can probably fix it and get several more years out of it, but I don't think new replacements are expensive from parts stores, and you can get a used one from almost any Ford product in a junkyard. They've used a compatible switch since the mid 70s. Some have extra features, but will work fine in your truck - I use one with the Autolamp feature from a ~'78 Town Car, but I'm planning on swapping again to one with Autolamp AND Autodim.
By the way, you told me about a week ago my engine sounded like it was revving like mad because my clutch fan was seized, and you were dead on. I replaced it and now it's smooth sailing.