Component Testing
WARNING: This page is about a different car, the 2008 GMC Sierra, 2008 GMC Cab & Chassis Sierra, 2008 Chevrolet Silverado, and 2008 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
With the stop lamp switch removed, use a DMM to measure resistance across the pins of the voltage circuit and the stop lamp switch signal circuit. With the stop lamp switch plunger released, the DMM resistance should display infinite resistance. With the stop lamp plunger depressed the stop lamp switch resistance should be less than 10 ohms.