Q: Left turn signal malfunction.

Check the bulb contacts, front and rear, and just make sure there is no obvious corrosion. Check the headlight harness connection to the headlight bulbs as well. From what you are describing, there is some sort of short or voltage aberration in one or both circuits such that when the headlight is added as a "load", the malfunction indicator responds as if a "bulb" is out of the circuit. If you can’t find a obvious external wiring fault, you will need to have the vehicle wiring diagram in hand from the Factory Service Manual (Mechanic will have, of course) because you will have to trace both the headlight circuit and the turn signal circuit and look for shorts, voltage drops and so forth. If the headlight switch on your vehicle is in the same assembly used to actuate the turn signals, obviously it is possible that something is going on in that "combined function" switch. The procedure on the type of fault you are describing is you have to trace the relevant circuits but this is certainly something a certified Mechanic could make quick work of and so what I would recommend request an electrical components not working diagnostic and that will identify the cause of the fault that you are experiencing.