Jerry:
I installed two different but similarly wired
tail/signal/brake lights on the rear of my 1948 Wee Wind.
The trailer also has it's original single rear taillight
(with two filaments for brake light and tail light) which I
wanted to have light up as a center brake light only when
the brake lights came on the two new rear lights.
I bought from J.C.. Whitney and installed their third
brake light logic circuit (part no. 13UG8974T for $4.99) for
the center brake light. Now it only comes on when both rear
lights are lit in brake mode but not when either light is
used as a turn signal. This logic circuit will help
simplify the wiring of your rear lamps if you keep the extra
equipment center brake light on your trailer. I realize
this hint does not answer your posted question, but it will
help with the center brake light when you rewire your
trailer.
Fred Coldwell
Denver, CO.