Everything works fine until I turn the lights on. Then the brake lights come on and the left turn won't flash. Right turn still flashes. No other issues so far. Cruise still works. Have checked the grd strap to the frame in back. Checked connections on tail lights on both side and connection along the frame in back. I see a lot of posts that are similar with probs with the multi-function switch. The truck is 2001 F-150 extended cab. Nobody has said anything about the headlight switch itself. Is this a possibility? I see one post with procedures for testing the MF switch and wondered if it is dependable. These symptoms came on suddenly and have not varied. No amt of shaking connections or tilting the wheel makes any difference.

 

Turn on emergency flasher switch and check all 4 corners. Check the T/S bulb for a burnt out filament. If it is an 1157 bulb, it has 2 filaments. Remove and clean both contacts.
If the bulb is good, the flasher relay (if so equipped) may need replacement.

 

Have checked all the light sockets for corrosion and damage. Four ways are the same(work fine until the lights are turned on). Thought I had narrowed down to the MFS and replaced that with no change except now with the door open I get the chime like I left the lights on without the key in the ignition. Pulled the light switch and got the same. Noticed the cargo light now takes almost an hour to go off when you get out. Still searching. Does this truck have separate flashing units for each side. Guess I'll get the schematic out.

 

Should have started out with the test lite. The four connecter trailer pig tail had shorted out inside the molded rubber. No sign of corrosion.