I have a 78 Ford F-250 with a puzzling blinker/headlight/brake light problem. I don't remember when the problem started but it has been going on for 2 or 3 months now. Today I have put in a new headlight switch, new break pedal switch, checked all the fuses, cleaned all the grounds, checked the bulbs( all bulbs work and are the correct bulbs),and flashers and looked for any broken or shorted wires. No trailer hookup or wiring for it. Everything has been checked accept the steering colum. The problem is this:

Everything works fine as it should until I turn on the headlights. With the headlights on the blinker lights on the dash glow and the blinkers do not work. With the lights on the right tail light goes off. With the lights on and I step on the break there is no right break light but the left break light works. Flashers work until I turn on the headlights.

 

sounds like a bad or brokenground,probably in the headlight circuit.I'd start there first.