My right brake light isn't working unless I have my lights on. Why won't it work w/o the lights?

My right brake lights only work when the daytime running lights or head lights are turned on. I checked the fuses and everything is fine, nothing blown. Why won't the brake lights work without the lights turned on? Its a 2005 Mazda 6 if that helps.

when a light "burns out" the filament actually burns in half. Once in a while, the filament can either "re-weld" itself together or perhaps "cross wire" itself internally and appear to work. It is remotely possible that the bulb itself has "reconfigured itself" and when the lights are on when the brake circuit is activated, the circuit works by cross wiring through the light circuit. Kin dof like a jumper wire works, only inside the light bulb. In cars, this can happen more frequently than at home as the car bounces and vibrates much more, and the sudden bumps and jolts can cause the spring like "broken" brake filament to bounce against the energized or "hot" filament of the tail lights, welding itself there, and providing an anternate completion of the circuit. You could check this with a continuity tester on the bulb contacts (this is assuming a dual filament bulb).
Re-design5 years ago
And if one is burned out it's a good idea to replace the other one at the same time.
good point
Kiteman5 years ago
It could be a dodgy earth-connection.
seandogue5 years ago
You might have a blown bulb. The brake light also acts as a running light on many vehicles, so it has multiple filaments. Check the bulb. You could just swap the left and right bulbs for a quick check to see if it's the bulb or the circuit.