This is going to sound crazy but our 99 f-150's blinkers will stop working but if you turn the headlights on, they will work for a bit then quit again. I have replaced the blinker relay and Multi switch (both OEM parts) with no luck. Someone said that it sounds like a ground problem that could be anywhere. It's hard to know when the blinkers are going to work and when they will not. If anyone can give me an idea where to start looking that would be great. When I can catch them in "not working mode" I plan to leave the key on with blinker engaged and start checking grounds. One mechanic said it could even be where the bed is attached to the frame. That sounds crazy to me but I am not a mechanic.

 

One possible cause/solution ...

https://www.f150online.com/forums/19...3-f-150-a.html