[IMG][/IMG][IMG]This is a test box I made when I was working on trailers regularly. I made a long set of leads that plug into the power input jacks and have alligator clips on the ends so I can get power anywhere. The fuse is inline with the power switch. The switches send power out to each individual circuit. With the switches in the off position, you can connect an ohmeter at the banana jacks and check the resistance of the circuits to check for opens and shorts.
It may be overkill, but this thing paid for itself many times over.