The mechanical guage has a tube on it that goes to a bulb that fits in the head, which is filled with ether. Since yours has wires, it's definetly the electric. It may have been swapped out at one time, not really a big deal. Mine even has an electric guage in it (not working, burned out on me). From what I can tell from the diagram, the dk green wire (L) connects to the Cold side, and the pink wire (R) goes to the hot side.
rustednuts, I was only speaking for the Chevrolet. I wasn't aware that GMC used electric, though.
