[VetteVet]

You'll have one wire coming from the turn signal flasher, usually purple, that goes into the turn signal harness plug. It feeds both the left and right turn signal wires through the turn signal switch.
What they probably do is connect the turn signal light wire to this purple wire and any time the purple wire flashes it flashes the turn signal indicator light. I'll check the wiring diagram I have and see if it shows that. I might only have the chevy diagram though.

Chevy wires the left and right turn indicators to the left and right front turn signal lights so when one of them flashes it flashes the corresponding turn indicator.