Unplug the gi connectors from the power supply. With all the lamps removed (make sure you got them all!!), there should be no continuity across the lamp holders. If there is cont. try this to locate the short: divide and conquer. Found the middleish lamp socket in the string and snip/disconnect one wire from it, this will isolate which half has the problem. Test which half now has the short on it. Then repeat the halving technique until you find the socket or the wires that are shorting.