Brake lights

I don't know of any sensor for the brake lights located under the driver's seat. My GM shop maintenance manual does not mention this specific condition however, it clearly says that if brake lights fail to work or work intermittently, the brake switch located on the upper part of the pedal must be checked, then the brake relay in the relay box under the hood and last the wire harness in the rear of the vehicle as wires may have corroded creating sporadic or permanent power shorts. It also says that bulb sockets are prone to create corrosion and should be checked, cleaned or replaced.

The park and brake lights are controlled by the BCM (body control module), the relay and fuses should be under the driver's seat. In addition to those fuses also check under hood fuses for BCM are all good. If all fuses and relays are good you may have a faulty circuit, likely wear and tear on the wire harness is causing an intermittent electrical connection for the brake circuit. I've seen on a GMC forum that the brake wire harness is considered poorly protected in some of their vans. Try asking on any of the following forums: gmtruckclub.com, gmcowners.com, gm-trucks.com, savanaforum.com for schematics or more specific help.

The GM shop maintenance manual that I have was revised March 2005, so it is possible that the BCM components were relocated by 2005 model year and, in 2003, they are still under the driver's seat as KenF says. The manual says that the relays for the BCM are located in the fuse box under the hood and are as follow: #11 Truck Body Control Module 4, #16 Truck Body Control Module 3, #18 Truck Body Control Module 2, #19 Truck Body Control Module 4, #24 Truck Body Control Module Ignition 1, #46 Truck Body Control Module-Accessory. Again, the manual may be referring to vehicles built after 2003 or 2004.