I have an 85 E350 6.9 van. I just noticed that when I turn my left turn signal on, the left rear running light goes out. When the turn signal is not on, its fine. A while ago I had wired in for trailer lights. I don't think I noticed this problem at that point...but I guess I'm not sure. Anyone know what could be going on?
p.s. When I say 'running light' i'm referring to the rear lights that stay on all the time when you are driving with the lights on....

 

Usually those symptoms point to a bad ground.

 

so i should look for ground connections somewhere in the wiring of the taillights?

 

Yes.
Also wouldn't hurt to look at what they did when they added the trailer light harness.

 

Possibly a wrong bulb. I have personally seen that cause problems.

 

i'm only getting back to troubleshooting this after a long hiatus. i replaced the ground that is closest to the lights (the one I had wired in when i put the trailer light connector in). the other grounds near the taillights all seem solid.

i tried undoing the trailer lights. when i try putting the original brown wires together i get no light. when i add in the brown wire from the trailer light hook up i get light again...but still there is no light when i turn the left signal on.

not sure where to be looking next?

p.s. i also tried switching the bulbs as per the other suggestion, but that didn't fix it either...