I can tell you it is not on the circuit with the dash lights, because it does not dim when the dash lights are adjusted down. Others can check me on this, but I believe I have non-altered wiring.
Also, I can tell you it goes on when the parking lights are on, so it is on that circuit.
Get power from anywhere on the parking light circuit, and ground it and you are all set .... unless you are looking for an "original" connection to the harness.
This is all free advice, so be careful with it.
Maybe the experts have something to add or alter from what I said.
Enjoy the ride !
Mark