You got it fella. It only needs a few millipedes of electrickery to fire the relay. So yes if a bulb goes or if things are a bit shonky, the relay fires and uses the power feed to illuminate the trailer lamps. If take my trailer (my quad bike trailer) and add it to the main vehicle you end up with 8 side lights and 12 indicators. All the extra stuff is essentially tapped into that main loom. If nothing else, put LED replacements in the trailer. You could say, Awe, I'd don't care about the trailer if none of the lights work, coppers aren't fussed. Personally I don't want to give anyone a valid excuse to stop me somewhere and start poking around. Especially if I am abroad and someone's looking for a little income support.