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Hi Folks,
I'm getting back to finishing my trike project. You can see something about it here:
My problem is how do you wire a three wire tail/stop/turn light to do all three with only two filaments
 
I'm afraid I'll need two separate/additional lights for either my brake or my turn lights.
Am I missing something? How come I don't find four wire, three filament fixtures available?
Thanks
Bill

This is the quick and easy down and dirty fix for your system Bill. if you had an old VW Rabbit with separate signals froms the brake lights, you had to install one of these converters to run trailer lights
. 3 to 2 wire trailer wiring converter 

 Basically, inside the box is a diode on each of the turn signal and brake light feeds to prevent back feed. You could achieve the same thing by obtaining 3 diodes and putting one on each of the left signal, right signal and brake light feeds and then marry up the brake and left signal down stream of the diodes to your left taillight and then the same with the right signal and brake feed to go to the right taillight and of course a divided feed from the taillight feed to each of the taillights.
Here's  a link for a DIY wiring with a parts list  3 to 2 wire DIY wiring with parts list
J