Looking for 12v trigger wire into amp

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I'm adding an outboard amplifier to my 2001 Tahoe to give the stock subwoofer a much needed boost. All the wiring is complete, except for the trigger wire that turns on the amp when the head unit is on. I found a wiring diagram for the Tahoe at this URL:

http://www.cox-internet.com/robertos/amplifier.html

The diagram shows an orange 12V power antenna trigger wire going to the amp, which would work perfectly as a trigger wire (as done in the installation described at the link above). Trouble is, there are about 6 different orange wires visible just behind the glove box, and I'm not sure which one is correct. Anyone know how to locate it?

I'd like to avoid pulling out the head unit if possible, and I'm hoping the trigger wire is easily accessible from behind the glovebox.