1) The other wire in the trunk is for the license plate light.
2) I would go ahead and change out all the instrument cluster bulbs for new ones and retest. Good time to put all new bulbs in there while you have it out. And you will have eliminated burned out bulbs from the equation.
This gets odder and odder. I was taking the gauges in and out, and one some attempts, all of the lights in the gauges light up, and when I turned the light switch on the dash they would go on and some locations, and off at others, flickering almost. It was strange. Took the light switch out and brushed off the connections, no dice. I havn't figured out what is making them come on sometimes and off others.
