There could be a problem in the turn signal switch itself, but if it is ground problem on the left light assembly, the lights shouldn't glow with full intensity. The path would be from the left turn light, either front or rear, through the light filament to the tail or parking lamp, through one of those filaments to the tail light wires and back through to the lights on the other side to get to ground. But, I repeat, none of the light should have full intensity in this scenario.
