Longhaul,
It sounds like you have it figured out. A wire should run from a key switched terminal, through the GEN light and then to the #1 terminal of DA plug.
That tab on the DA plug should only show voltage when the key is ON, not when it is off. When you turn the key ON, the GEN light is powered and finds a ground through the internal regulator. When the truck starts, that terminal now generates voltage, the ground is lost and the GEN light goes out.
I think this is how it works.
Stuart
