All 3 bulbs were out in my left light assembly, I replaced all the bulbs (I'm pretty sure I have the right ones, checked 4 times now). When I have my headlights on the blinkers will blink but not bright enough to be seen. If you look you can see one filament flashing, it isn't that noticeable. When I cut the headlights off, the blinkers work just fine.

I have power to all lights.
I have the correct bulbs.
Blinkers work fine when headlights are off.
When headlights are on, 2nd filament flashes but not noticeable enough to be considered a turn signal.

What can I do? I'm stumped!

 

I would start by checking the ground on that side. I think it's on the radiator support near the back of the headlight.

 

I'll check the ground in a few but I found a thick black and red wire that isn't connected to anything. Here is a pic. The red wire was cut and tucked up somewhere, one set is the trailer harness of course and the other cut wire is the wire bundle that the trailer harness was spliced into.