I've been kicking this idea around, too. I got stalled at the wiring for only a brake light...i guess I have to run stop/tail/turn, or at least stop/turn. With the shared bulb makes it goofy.
I suppose I could add a relay to reverse the polarity of one of the signals, and use that to ground the high mount? That way it would only light when green and yellow wires were both energized...i think...maybe
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