Turn signal question for the wiring gurus!!

[BurnoutNova]

This is mostly a trivial question, but I was discussing wiring a painless, or other aftermarket harness into a 67-72 pickup with a friend.

The topic of the marker lights came up, and the way that most aftermarket harnesses seem to show the wiring of the turn signals and marker lights suggests that they wont work just as the factory does where they alternate blinking from the bumper/grill light to the side fender light.

So... my question is just how does the factory wire the lights to make them alternate? Is it that they run the power to the ground side of the side marker (to turn it off by putting power to both sides of the 194 bulb, so it alternates blinking that way? If so... I don't understand how it works when the running lights aren't on?

Any speculation would be great to hear!!!