Front left turn signal will not work when headlights are on?

Let's start by checking the ground which is located at the left side of the engine compartment (G112) if that is okay is sounds like you have a TIPM going out which is common on these cars. They are easy to replace because they self-learn. Here are the instructions on how to replace the unit, plus I have included the blinker wiring diagrams so you can see how the system works. Check out the images (below). Let us know what happens and please upload pictures or videos of the problem.