The self resetting relays sounds like a nice option.
The reason i don't just wire the car/trailer directly is because in many cases the trailer is initially wired for a specific vehicle which may/may not have been wired correctly. Most of the usefulness of this device comes from when we need to borrow or a trailer from, or loan one out to, someone else. Rewiring their trailer to standards might be the 'right' way to go about it, but it's not really an option as it would often be wired incorrectly for their original application (plus, rewiring a trailer at 20 below is what i'm trying to avoid in the first place).
In general, there is already a harness that goes between the vehicle and the trailer which, if wired correctly, delivers power directly from the taillight circuit on the vehicle to the taillight circuit on the trailer. it's this harness that i'm trying to tap into or adapt, and adding another dedicated power line would be somewhat inconvenient.
Would you happen to have an example of how I might do one or two of the wires, using the input line as both indicator and line voltage to pass through to one of 4 output wires? I'm a little weak on the technical details, but if someone can point me in the right direction, i would be grateful for the help.
Thanks,
-Dan