do i have a short? or just short on knowledge?

Putting the dash back together in my 66 Chevelle Malibu and have come upon a grounding issue, i think. I upgraded the idiot lights to gauges, and therefore ran wires for the new ammeter, Before reinstalling the dash i have tried to get the dash lights to work (the 5 backlit lights that are on Grey wires). I thought that i should be able to turn on the lights, and simply put a ground wire to the metal part that holds the lights to the back of the dash. This did not get the bulb to light. However when i put a positive lead to what i thought should be the ground connection, the bulb lit up.
i tried disconnecting all of the new wires that i installed for the gauges (and Vintage Air AC), and still get the same results. I am assuming that the back of the dash (metal ) is supposed to be negative ground, and that i must have a short in the wiring harness.

Any ideas or info would be greatly appreciated.

Paul in CT