Checkmate 1200 is a good, basic light tester for trailer light systems that can also serve double duty to power anti-lock brake systems. At this very reasonable price, why build your own?
Auto-reset circuit breakers with short circuit warning light.
Flashing turn signal feeds
One-piece, plyethylene rugged carrier stands up to rough use, shop dirt and weather
Small profile enables you to fit in tight spaces
Carrier accomodates regular car battery