On a recent trip, I had a problem with an intermittent short in the running light circuit that I could not find, and needed to drive home in the dark. Running short on fuses, I hit upon an elegant solution: I wired a 21 watt brake light bulb in series at the umbilical connector. With the filament not glowing, the resistance is very low, and the LED tail lights and low wattage side lights ran near full brightness. As I bounced along the road and the short would occur, instead of blowing yet another fuse, the light on the tongue would glow, causing its resistance to increase so that the lights winked out but the fuse didn't blow; when the circuit un-shorted, the bulb on the toungue would go out, and the running lights would be back. A light bulb in series with a circuit works as a ballast resistor, meaning it increases its resistance with higher load. The wattage of the ballast bulb must be somewhat higher than the sum of all the lights in that circuit to allow close to normal voltage to the light when the short is not present.
This is more protective to the wiring than a circuit breaker, which allows full current until it cycles off. On a trailer circuit with long wire runs, there is a greater risk that the wires could get dangerously hot without cycling the breaker. This is especially true with higher amperage circuit breakers; cars come in to my repair shop all the time with too-big circuit breakers and melty-looking (but not burnt) wires.
I thought I'd pass it along, as it may help someone else out of a bind.
It seems I love the mountains and deserts more than my friends do. I sure miss them!
1971 Streamline Imperial project "Silver Snausage", 1985 Coleman tent trailer, 1964 Little Dipper, 1975 Northwest "Proto Toyhauler", 2004 Harbor Freight folding, still seeking my Airstream.