Left turn signal issue.

Doubtful it's the switch. Try swapping the 4x flasher for the turn flasher.

There are only 3 sources of power "to" the signal system, barring shorts/ wiring faults

1--brake light switch, provides power into the column switch only when the brake pedal is depressed. Easy to eliminate that as a source

2--turn signal flasher power, so power with key in run comes to, through the flasher and to the TS switch

3--4x Flasher switch, dash mounted is a 4 terminal switch. With the switch off, all 4 terminals are isolated. With switch on, all 4 are connected together. 4x flasher power comes to one terminal, and one terminal goes to the brake light switch line, which then feeds through the TS switch to both rear lamps. The other two terminals go to each, left, and right, front lamps

So you have to ask yourself, "what is lighting" those lamps?

You can make sure the brake light switch is off

Make sure the park/ tail lights are off

You can reach around and unplug the 4x flasher switch

That leaves only the TS flasher, and a wiring harness fault.

Try a different flasher. Either swap the two around, or buy a new one--they are not expensive.

Pull all bulbs and check them