I have a 77 f150 and recently noticed the passenger side tail light and the side running light doesn't work when the lights are on. The blinker works and I put switched the bulbs from the other side and the bulb is good. I have also tested all the wires and the brown wire doesn't have any power. Any ideas? Thanks

 

Trace the wire back, starting at the bulb, checking for broken wires, loose connection, and corrosion/dirt in the connections. This will be time consuming but you will find/repair the problem doing this. Keep checking for power at each connection, when you find power you know the problem is behind you (between there and the last connection) in the harness

Hope this helps