Rear brake lights

Good afternoon.

It sounds like you have a bad ground. when you have the running lights on and then press the brakes, if you have a bad connection to ground, the lights will not work.

Look at the wiring back at the light and make sure the connectors are clean and tight. there should be a ground wire coming off the light assembly to inside the rear area. check that connection for any corrosion.

Roy

https://www.2carpros.com/articles/how-to-check-wiring