Q: 3rd brake light on

Hello. From what you describe it would appear that your vehicle is experiencing some sort of electrical issues. If only the third brake light stays illuminated, not the left and right side brake lights, then that is a sign that there is an issue with the third brake light circuit. I would verify that the switch that was recently replaced is working properly as it’s failure could be what is causing the issue to reoccur. If the light stays on even with the car off then that is a sign that the light is getting power somehow, perhaps due a fault with the switch, or an electrical short. If the switch is good then I would move to looking at the third brake light connector, socket, and wiring to see if any of them may be damaged and possibly causing a short. It may also be possible that the short is somewhere away from the brake light assembly, perhaps somewhere in the harness in the interior or on the undersides of the vehicle. If the light is staying on then you can prevent the battery from draining by disconnecting the battery when the car is off. This will, however, require you reconnect it every time you wish to start the vehicle. As electrical issues like these can sometimes be difficult to diagnose, I would recommend having a professional techician, such as one from YourMechanic, visit your location to diagnose what may be causing your third brake light to stay on.