Found unit #2 with a blown fuse . Isolated the air handler and tested it . Then I isolated th airhandler with the thermostat and tested it . Then I added the safety switch and tested . Then I added the out door stat wire and it blew the fuse . Then I removed my wire from outdoor and tested and no short . So our low voltage short is located in outdoor . I looked over wires and did not see any burn marks but because of the snow falling I could not go into detail . Costumer has heat with Aux and we will need to come back to correct the short in the outdoor. Spoke with Alex and he said no charge for my visit but when we come back everything will be billable and I made costumer aware of this .