2000 chevy express 1500, when I press the brakes the left rear brake light goes out, the center and left still works with lights on or off. when I turn the lights on the left rear is dim?

wirebrush the black ground connection to chassis...clean the connection to the block...inspect the battery for corrosion...hose clean if stuffy~ inspect the bulbs...and may as well change the double- filament ones out 2156 used to be in the day...now??? if you've got the lens removed, change the bulb and use something like fantastic to bring the shine back and the dirt out from the rear lens assemblies...'round here the spiders get in there...don't know what they are eating...electrical insulation?~

sounds aw though the left is not getting sufficient amps to light as brightly as it should. a bad ground or a pinched hot feed wire could be the cause. you can use a clampon volt-ammeter to measure the amp draw to both the left and right rear brake lights, you may find as I did that the lower light level in the left bulb was due to a pinched hot wire not allowing a full 78 amps to flow to the rear lamp assembly.

78 amps? 10 amps at the most...you could arc weld with 78 amps....sheesh~

7.8 amps must have just missed one tiny little dot in that~

There is definitely a ground problem, and it is very common indeed with these trucks. It is likely to be the ground contact in the socket, since it is only the one side that is acting up. Were it where the ground meets the body, chances are the light would go out at other times, such as when you put the opposite turn signal on. Part of the problem is that all the car companies are using very thin wires, and the copper strands are breaking where the wire meets the socket. The after market sockets have a heavier wire, so tend to last better.