Turn signal issues

[Longview Energy]

I have a 2007 Freightliner Columbia with a 14L 60 series detroit. My problem is when my headlamps are on both of my turn signal indicators are illuminated. The right signal works properly when switched on however the left signal looks as if its going to work but doesnt and no left signals for the truck or trailer activate. Now when the headlamps are off and just the markers are on the left signal works as it should. My fuses are all good. I borrowed a instrument panel fromnone of my other identical trucks and the problem stayed with the truck so Ive ruled out the instrument panel as the problem. Anyone ever have this issue?