During the day both turn signals work perfectly. When I turn on my headlights, the left turn signal (front and back) stay constantly illuminated and will not blink when I set the signal to the left. The right signal still works properly. Which component is failing/getting mixed up?

03 suburban

 

Body Control Module

 

Make sure the left headlight ground is clean and good. Signal light may share the same ground as the headlight causing weird feedback. Do this before going the BCM route.

 

interestingly, the left park/turn/marker lamp shares a ground with the RIGHT headlamp.

 

Does this mean I should check the right headlight socket for corrosion, how can I check the headlight ground?

 

Yes. follow the harness. See where it's grounded to the body/chassis

 

Followed the harness. The side marker wiring joins with turn signal lighting. The turn marker wire about 4 inches back from the bulb was crushed between a piece of plastic and the chassis. Once I prayed it free, put everything back together, problem solved.