Tail Light Stuck On

My left tail light is stuck on for some reason. I noticed on some late night driving where the left looked bright like I was braking but the right looked normal. Any ideas? The right seems to function fine. I replaced the brake tail light just a little bit ago.

 

Got an aftermarket
Brake trailer light controller? They’ll do weird things when they go bad

 

The dual filament bulbs can only be installed one way, but sometimes it's possible to force it in 180° off. If you did that the brighter filament (brake light) would be on when dimmer taillight should be on.

Other than that these problems are usually related to loose or corroded wires/grounds. Trailer connector are notorious for corroding and causing problems.

Another thing to check is the contacts at the bottom of the bulb sockets. They can melt and cause shorts between the terminals.