2002 Chevy Tahoe taillight inoperable

I've got a 2002 Chevy Tahoe with the towing package, and the passenger side tail light isn't working, but the bulb lights up just fine when you hit the brakes.  Replaced the bulb, replaced the socket the bulb plugs into, and have traced the wires and can't find any dirt caked into the connection at the circuit plate.

None of the fuses are blown, either in the cab or under the hood, and the driver's side taillight works just fine, so I'm thinking it's a problem close to the actual light at the rear, rather than something up front.

Any idea where the problem might be?

Which of the two wires on the 3-wire light socket controls the taillight signal, and what should the voltage be?

If I'm not reading any voltage on the wires, what should my next step be?