My left turn signal activates the 4 way flashers, Chevy replace the multi switch but the problem persists. Any ideas for a resolution?
So, left flashes left and right. Does right only flash right, or does it flash left and right too? If the latter, then you may have a short between left and right somewhere. Do you have a trailer harness added?
Try replacing the left stop/turn lamp. Yea, I know it lights up, but just put a new on in there and see what happens.
I think JEM is on to the trouble especially if you notice that the front lights also work when applying the brakes. If this is the case then the left side brake light bulb filament is touching the the running light filament.