When the new 12V harness was put in, were the headlights put on relays? If so, maybe the low beam relay croaked.
I would unplug on of the headlights, check your resistance to ground, then check the voltage of each of the terminals with the headlight switch on and the dimmer in each position.
You could also have a problem in the dimmer. They get a lot of abuse and are not in the best postion. Meausuring the voltage at the terminals will tell you if you are getting voltage (full or reduced) there.
Either way, that will tell you where to look next.
A good Tech Tip
