Original-equipment connection point would have been at the back of the ammeter, on the same terminal post as the supply wire for the ignition switch and the Bat. lead from the voltage regulator. Be aware that the connection will be unfused, and shorted cigarette lighter wiring has been the cause of a LOT of under-dash fires. Even if it's not purist stock, I'd recommend a 30 amp inline fuse in that wire. To make it look a little more original, use the round black holder for a glass-tube fuse.
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!