Sounds like a bad headlamp switch
I am having troubles with the right turn signal on my 1975 Ford F-150 super cab pickup. During the day with the headlights off, the turn signal seems to work fine. At night, if I have the headlights on, the turn signal does not work. The brake light and running light work at night (and during the day), but no turn signal. Any thoughts on what the electrical issue could be?