Turn Signal Indicator Fault?

[Gunther]

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?