Very neat idea Richard, a great way to check individual lines. Thanks you for sharing this, will save on fuses in the future.
I should have mentioned that you need a DC circuit breaker, also that the unit can actually be connected to power on either end. With both power and ground (rather than a 12 volt test light), you can remove bulbs and check them right at the light.and also check both power and ground in the bulb socket. Its been a few years since I used this, so I forget a few of the uses.