Left turn signal issues.

Right at the socket or the socket ground wire if it has one

The way this works is, with the socket UNgrounded, you have set up a series circuit.

With the park lights on the "juice" goes through the park light filament, connects to the signal lamp filament at the GROUND point at the socket. Since there is no ground, the current then goes through the signal lamp filament, but there is no ground there, because the signal lamp is "unswitched" at the turn signal (signal lever in center.) BUT the signal lamp filament is hooked to the DASH INDICATOR which IS grounded.

So the "juice" goes from ground, through the INDICATOR filament, up to the SIGNAL lamp filment, through it, to the "UN" ground and the socket, through the PARK lamp filament, and back to the light switch on on to the battery positive (Juice "electron" flow is from neg. to positive.)