does the trailer have a battery in it? if so just wire up the relay and switch to that battery, then you could turn on the lights even if the truck wasnt hooked up, you mentioned interior lights and could also run them too. or just put relay in box with trailer harness connector and tap into the...
just wanted to say hello to everyone. ive got a little knowledge on diesels from the Caterpillar truck shop in san leandro, CA. a semi truck on a dyno under full load is a cool thing!!!!