Check the headlight connections and the wires going along the radiator's cross brace thoroughly, 'cause that's the only area that's changed on the car really. You're probably going to have to tear into the instrument cluster and look closer if you can't find the problem. My guess is a wire crossed somewhere. It's not scary as it sounds, though. You need a phillips screwdriver, ¼-inch ratchet, a 3/8- or 5/16-inch socket with extension, a ¼-inch socket, and a T15 Torx bit...that and a little patience. Good luck.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch