You are correct that the prongs that hold the socket to the dash are also the routing for the ground connection. Are you sure you are getting power through the gray wires? Is the fuse blown? Do you have a multimeter? If so you could do the following. With the multimeter on DC volts and the headlight knob in the position to turn on the lights take the positve probe and touch the positive connection in the bottom of the bulb socket (inside the socket) and the negative probe to a good ground. The meter should show something near 12 volts. Meters can be had inexpensively and are v ery helpfull. Also make sure the headlight knob is not turned all the way to the right as to dim the dash lights. There needs to be a ground wire run to the metal structure of the instrument lights which I assume you have from your seat track. I think you could find a closer ground source but if thats a good ground i guess its ok but I would clean off that paint or soundproofing around the immediate area that you have tha ground connection connected to.