How do you intend to switch the turn signals on and off? It's possible to accomplish what you want to do with the existing 2-filament tail/brake light bulbs, with some very simple rewiring- - - -no diodes required. It will require a turn signal switch with a turn signal/brake light disctiminator circuit built in. That's not going to happen using a year-correct turn signal switch for Charlie- - - -because they don't exist! A Grote, Signal-Stat, or other big rig turn signal switch clamped to the steering column will do what you want- - - -switches don't care what voltage they encounter.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!