2004 chevy 1500 brake light problem

The third light is on a different circuit, but is attached to the brake sensor to send a current to the light bar. Check for a lose wire or broken wire leading to the brake lights. Also try a new bulb, as the other lights will work even if the brake bulb filament is k-put.

every time I replace one side brake light the other side goes out. what is causing this? how do I fix?

Lola_racing: What you have appears to be a short or bad connection Check all the wiring for breaks or cracks. Make sure the connections are free of other wires. Perhaps the connectors for the bulbs themselves are bad. If you have someone with electrical knowledge in 12 volt circuitry, have them check for voltage and if there may be a short or lose wiring. You may have to work all the way back to the circuit breaker.