Did you just install the screw or did you take the booster off so that you could verify that the plug was installed correctly? Just wondering if the bulkhead connector is still not mated/seated well.
Other possibilities would be the dimmer switch. They can go bad but as I mentioned before, when something suddenly quits working, look over what's been done recently. You've had all the plugs on the light bulbs off testing the bulbs so double check to make sure they are all plugged in correctly and plugged in well.
The dimmer switch simply switches the voltage from the tan wire(low beams) to the light green wire(high beams-all 4 lights). I may have called the tan wire brown the other night. So the fact that you have no lights at all when you click the dimmer switch to "low beams" makes me think you've still got a bad connection on the tan wire for the low beams at the bulkhead.
As I said on the phone, the inner lights(high beams) get their voltage from a light green wire that simply jumps over from the driver side outer(low beam) light bulb. So if you have the two inner (high beam) lights coming on, then you're getting voltage for all 4 high beam bulbs to the front of the car. At least to the driver side outer bulb. So it sounds like you may just have a bad connection at the bulbs as far as the high beams go.
Again, if you have no low beams at all, it's most likely a bad dimmer switch or a bad connection at the tan wire in the bulkhead.
