Localizing a Short

I have trailer with a short in the marker lights -somewhere- between the plug and the 15 lights that run off that circuit. So far I've figured out they didn't believe in grommets and the light bases are riveted to the trailer. Before i start breaking -more- plastic light bases and digging for a rivet cutter, is there a trick i don't know about for localizing shorts?

Localizing a Short