Wiring Question

Have a question on wiring. I have installed a complete Ron Frances Harness in my 65 mustang. I am now ready to check to see if every thing works. On the instructions it said to make sure the ignition switch and all accessories are off . It said to connect to the battery positive cable only , Do not connect the negative cable. Then it said to take your test light and connect it to the negative post on the battery and the other end to the body ... If the light comes on you have a direct short somewhere, well guess what mine comes on. Now the question is what or how do I find the short ? Any ideas thanks for the help everyone.

Wiring

Ok, had my wife hold the test light and told her to let me know when the light goes off. I did not get to any fuses just started to move some wires out of the way when she said the light went off ?????? I checked it and it was off so I tried the ignition to see if the engine would turn and it did. Ok I checked the heater switch and it works, the wipers worked and then i pulled out the lights and the dash lit up however I checked the tailights and the one on the left was bright and the one on the right was dim I reached over to twist the bulb and it blew the fuse. I did some checking and I had failed to ground the tailight assembly near the bulbs. I could not find any thing on how to ground them so I went from the tailight housing bolt to the rear trunk bracket do you think this would work also do I need to buy a grounding strap from the body to the motor someone told me I need one any ideas on what I should do next