Left front signal does not flash

Some of what you are describing are the classic symptoms of a broken ground circuit. There are some voltage tests that could confirm that, but given the history, (thank you for including that), start by inspecting the wiring for the affected bulb. I have not dug out a wiring diagram yet, so for now, we will assume you have the typical front turn signal and running light in the same bulb. When that is the case, with a bad ground, the signal bulb will act differently when you turn on the running lights or head lights. Also, when the left bulb doesn't work properly, it will go out completely when you remove the right bulb from its socket. With that bad ground, current wants to go through the left bulb but cannot find the return path to ground. Instead, it travels over to the right bulb which is connected to the same circuit, then goes through it and then to ground on the right side. Removing the right bulb removes the path for current, so the left bulb goes out.

Follow the wires from the socket. One of them must be bolted to the car's body sheet metal or a metal structural part of the car.