Turn Signal Indicator Fault?
 

I just finished a major overhaul on my lighting system (new front, rear & intermediate rear harnesses, new dimmer & turn signal switches, upgraded from cluster without gauges to one w/ gauges including new underdash harness, even replaces flasher) and the left turn indicator is lit anytime the headlights are on.
The signals & hazards work perfect and the indicator flashes when they are on.
The underdash harness (only part replaced not new) was in great shape & I checked every inch of it before installation.
Anyone have a clue what's causing this?

it's usually a bulb , it's either not twisted all the way in it's socket or a filament is burned out

Another thing that can happen is thebulb shiel is out of location and lights from your dash shine in, lighting the indicator.

Have you tried turning the dash lights off as a way to check this?

Jim