I just got a PowerProbe 3 tester. Think of it as the ultimate test light. First it comes with hookups for both alligator clips and cigarette lighters to supply power to it and it has an auxiliary ground at the tester so you don't have to search for a good ground... When you test circuits it displays polarity + or - as well as voltage. There is a toggle switch that allows you to connect the probe to positive or negative.
I just used it to hookup the 3rd brake light in my new camper shell top on my truck. It was sooooo easy...
Just wanted to let everybody else know it exists...