Originally Posted by mgburg

Those converters use diodes to "steer" the voltage from the input circuit to the proper output circuit.

Think of it this way...

If the two Input circuits (Stop/Turn Lights and Running Lights) are used to power one Output circuit (Stop/Turn/Running Light), you have to have something BLOCKING the activated signal from coming back and interferring with the source of the Input circuit's activation...

Diodes let electricity flow in one direction and impede/stop it from coming back to where it came...

Diodes also "open" or "short" - therefore, troubleshooting a black block of epoxy with input/output wires is pretty touchy, to say the least...unless you understand the basics of that type of circuitry, or you're able to read a schematic.

Generally, once that block of black starts acting up...ditch it and either modify the trailer light circuitry to mimic the towing vehicle's own, or just pitch it out and get another one...and if you're traveling great distances, buy one or two extra, just in case the one hooked up as you leave your homestead decides to "vacation" itself...