With a test light it is easy enough to figure out which is the 12V and which is the ground. If you go with an incandescent bulb - it really won't matter, but if you go with an LED like this one:
TARGA LED LICENSE PLATE LIGHT - WHITE LED - Targa
you need to have the current going in the right direction. Simple enough to do - if you're in the Waterbury area shoot me a PM, and I'll help you out. Good luck