[f5fstop]

From the GM wiring schemantics, component connector end views.
Left rear taillamp socket.
Pin A D-BU circuit 18 Left Rear Stop/Turn Lamp Supply Voltage
Pin B BN circuit 2509 Left Rear Park Lamp Supply Voltage
Pin G BK circuit 1450 Ground (two wires).

This dark blue wire goes directly to Splice S245, located under the sill molding. At this splice the dark blue wire is spliced to two yellow wires. One yellow goes to the trailer harness, the other goes to Pin B1 of the BCM. Since this circuit is called the brake/turn circuit, does it or does it not make sense that if you tap into this wire with the mirrors, you will have power to the mirros whenever the brakes as well as the turn signals are applied?
The BCM has total control over this circuit. It closes both circuit to apply the brakes upon input from the brake switch. If a turn signal is turned on, the BCM overrides the brake input to that side, and turns that particular circuit on/off to make it a flasher.
Front circuits work the same, except replace the word BRAKE with the word DRL and change the wire colors.
To say to add a few relays might make it work, but at this time I have not the time nor the care to figure this out. However, to wire in a few relays, when you can tap into the dedicated circuits that go directly to the I/P signal indicators, makes your reasoning ludicrous.
So, tap into those wires, add in a few thousand relays and go for it.
Not sure what wiring diagram you are using, but I'm referencing the actual GM wiring schematics. Can't say they are always correct, but, on this circuit, they are.