Noid lights aren't very good for diagnosing electrical problems.
Basically, the headlight circuit works like this:
The positive terminal of the battery goes to the combo switch. The combo switch goes to the fuse box. At that point the circuit branches through both fuses. Typically it uses a red wire for the right and a red wire with yellow stripe for the left (Dowload a FSM for your car to verify wire colors.). There are two wires that come off each side. One is red with a white stripe (high beams). The other is red with a black stripe (low beams). Those wires go back to the turn signal as return paths for the high and low beams. When you switch from high to low (or low to high), you are changing which wire is in contact to ground, which goes back to the negative terminal of the battery.
This is why it's handy to have a multimeter...
You should have 12VDC to ground at the headlight plugs, but only at the red wire or the red wire with yellow stripe. I suggest testing with both headlights removed to prevent reading the voltage that is backfeeding through a headlight. The backfeed could be enough to light up your noid light, but a meter would probably show the voltage drop. By the way, make sure you are touching a solid ground with the negative lead of a voltmeter. That could have been why your noid light didn't work on the one headlight.
You are likely missing the path to ground at the steering column. The grounds are connected between the turn signal and the combo switch, and the true ground wire goes out through the combo switch. It fits with pretty much everything you described. The BE section of the FSM has all of the information you need for this. All it takes is reading and understanding it, and you'll be in great shape.
Basically, the headlight circuit works like this:
The positive terminal of the battery goes to the combo switch. The combo switch goes to the fuse box. At that point the circuit branches through both fuses. Typically it uses a red wire for the right and a red wire with yellow stripe for the left (Dowload a FSM for your car to verify wire colors.). There are two wires that come off each side. One is red with a white stripe (high beams). The other is red with a black stripe (low beams). Those wires go back to the turn signal as return paths for the high and low beams. When you switch from high to low (or low to high), you are changing which wire is in contact to ground, which goes back to the negative terminal of the battery.
This is why it's handy to have a multimeter...
You should have 12VDC to ground at the headlight plugs, but only at the red wire or the red wire with yellow stripe. I suggest testing with both headlights removed to prevent reading the voltage that is backfeeding through a headlight. The backfeed could be enough to light up your noid light, but a meter would probably show the voltage drop. By the way, make sure you are touching a solid ground with the negative lead of a voltmeter. That could have been why your noid light didn't work on the one headlight.
You are likely missing the path to ground at the steering column. The grounds are connected between the turn signal and the combo switch, and the true ground wire goes out through the combo switch. It fits with pretty much everything you described. The BE section of the FSM has all of the information you need for this. All it takes is reading and understanding it, and you'll be in great shape.
