Just a wild shot in the dark, but something that happened to me. Does your mustang have an aftermarket radio? I found my fuse to my dash lights blew when I turned on my headlights because the radio was wired wrong. Something about the hot wire that dimms your radio light when the headlights are turned on, was actually wired as a ground. Probably not your problem, but just something to check out.
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1985 Mustang GT