Who knows about turn signal wiring?

mopar56 said

I wonder what would happen if I took the 12v output from the brake light switch ( which currently goes direct to the rear lights) then spliced that into the two supply wires to the rear lights leaving the column to the rear signals? In theory that would interrupt the supply through the flasher? I think?

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If the white wire is "bundled" with other wires, it probably isn't the brake light input wire to the turn signal switch. I don't believe you can take the 12V brake light output and splice it into wires feeding the rear tail lamps. You'd be bypassing the turn signal switch and feeding the tail lights with the brake output..... which you already have working. The turn signal switch makes the determination which hot wire (brake light or flasher) feeds the tail lamps.

Dana suggested you make a continuity chart, which is a good idea. You have to know for sure what you're working with. If Dana's suggestion doesn't work, I'd be tempted to pull the steering wheel and look at the turn signal switch, draw a diagram and figure out which color wire does what.