Thanks for the replies. I guess it is possible that it could have been for an antenna. My Grandfather bought this truck new in 1981 and he did use it for pulling a camper, but I don't ever recall them having any kind of antenna that would have attached to the truck. They had an old analog that raised from the camper itself but I suppose that it could have been an option that he requested when he bought the truck originally.
As for what's at the other end, I do not know that because it goes right into the firewall and I haven't attempted to thread it back through to see if I can figure out where it would go. It's right in the section that has all the components for the a/c system and I'd probably have to pull everything out to get to it to thread it back through and then find where it terminates. I may have to do that eventually though.