so test light to positive on the battery. should only have 1 wire on it. not a meter, but test light.
the ground connected to the battery on your harness. no positive cable connected.
Now you should be able to touch the test light to the positive cable and say with the key on or a door open, (assuming your switch works) and the light should light up. It completed the circuit and is drawing power, but limited to your test light. Your interior lights shouldn't have enough power to come on. (at least I think) worst case you pop your test light.
Doing this with a meter might burn it out. or blow the fuse and I think you would have to do amps.
Anyway, now if you check any other circuit it should do the same. Anything that completes the circuit to ground, either by dead short or a switch turned on to ground, will cause the test light to come one.
But you have to be sure you have enough of the circuit there to test. As long as the engine to body ground is there it should work on any dead shorts to ground no matter where they are. You can test it by touching the test light to the body or bare block and see that it lights up too. That means any wiring exposed and touching the body or block will cause it to light up too.
IF it doesn't then you don't have a complete ground circuit and you will have to find out why. I can't imagine that won't work, but let us know if it doesn't.
