My left turn signal and tail lights don’t work when I turn on lights.

Any trailer harness on the vehicle? If so, check to see how that wiring may be affecting the rest of the vehicle and concerns as described. Have you checked the light sockets for power when tail lights are on and signals are engaged? Have you traced wiring back to find out where the power stops if there is no power? A wiring diagram is key here, besides test light or voltmeter / ohmmeter. When did the problem start, was there anything that happened prior to the concern starting?