I don't know Isaac,

It looks like the delay of the relay would still allow the iButton to take a pretty big 24V hit. I don't think the parasitic power cap in the ibutton could take too many of those.

Just thinking out loud here.

If on the cab side, the current to the reverse lights was monitored with a hall effect current sensor. And, on the trailer side a normally ON solid state relay or FET was placed in series with the light bulbs. Then the lights can be turned on and off (fast enough not to interfere with the lights being on) by a PIC sending serial data. The current sensor in the cab would pick up that signal easily for the receiving side.

Then every time you put it in reverse, the 24V will power up the PIC and it'll transmit what it's attached to. Same thing could be done with brake, turn signals or running lights.
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