Hi-I have a 2001 Supercrew and having a issue with the left turn signal not working when headlights are on,they work fine when lights are off. Any ideas on what might be the issue.Thank You

 

If I was to guess, I would suspect a short in the wiring on that side, your parking lights come on with the headlights. I would look at the wiring and bulbs both front and rear to see if there is exposed wire or a funky bulb.

By chance have you replaced a bulb or installed LED bulbs recently?

I had an issue with LED taillight bulbs, where they worked fine with the lights on, but off if i turned a signal on all the parking lights would flash. It turned out to be the drivers rear bulb was shorted out. I ended up just using incandescent bulbs instead and returned the LED's. I figured the LED's were cheap and couldn't handle the on and off of braking along with turn signal use since I drive with my lights on as well

 

Thank you for your input, I finally found the problem. I have a light bar under tailgate and I unplugged it. Now turn signal works with lights on.I think it has a bad ground:

 

Glad you got it sorted out.
I have a buddy who installed lights there as well, gravel and debris getting trapped between the tailgate and box ended up wrecking his lights.:thumber