It's probably a bad ground connection. I'm assuming the driver's side low AND high beam comes on with no problems, right? The ground is that little eyelet screwed into the chassis right above the headlight assembly. Kinda cheap way to do it, don't you think? Anyhoo, remove the sheet metal screw, shine up the connection (both the eyelet and the chassis itself) with a little sandpaper, and bolt it back down. Hope this helps.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch