The tail light wiring harness in the cargo area is not protected very well. As a result it may become damaged due to normal wear a tear. This damage can causing some or all of the rear lights to stop working.Google+
Car Problem Reports
Chevrolet Express 3500 Tail Light Wiring in Cargo Area May be Damaged
Chevrolet Express 3500 Problem
Average Mileage: 126,729 mi (55,950 mi - 289,940 mi)
My Chevrolet Express 1500 (chevy express 2500 or 3500 has the same fixture) has been experiencing a problem with the rear tail lights, same issue as many have mentioned by several others here as well. Although I have no idea if it is related, but after having a hitch installed on my 2002 Chevrolet express, the lower brake light (both left and right) stopped working when the headlights were on. They worked fine when the headlights were turned off.
After a visit to a local mechanic I was told that the rear light wire harness / wire socket was damaged and needed to be replaced. The price would be $550 to replace both sides!! After a 5 minute search I found replacement parts on ebay for $35 (Dorman 923-015). Installed in about 20 minutes and got it working again perfectly. The assembly is very easy (never tried anything like this before, but could not be easier). Saved over $500 in less than a half hour.
Visitor, 2001 Chevrolet Express 3500, 168,950 mi
tail lights not working when night lights are on.
Visitor, 2007 Chevrolet Express 3500, 55,950 mi
Rear left turn signal not working. Bulb is good, all fuse are good.
Visitor, 1997 Chevrolet Express 3500, 80,000 mi
Tail lights and blinkers started to malfunction. Problem was a bad ground and fixed by running additional ground wires from light sockets to frame.
Brake lights work fine when the running lights are not on, butonly the top brake light works when the running lights are turned on.
Visitor, 2001 Chevrolet Express 3500, 99,700 mi
brake lights work fin till you turn on headlights then they do dim only to leave third brake light