Brakelights

Clean your grounds to the engine and the lights.

Have you modified vehicle wiring in any way? Like trailor wiring?

Maybe this will help? There was an issue with poor contact in the printed circuit board that houses the bulbs. Open rear gate, remove screws, pivot lense assembly outward. Disconnect connector. Remove bulb holder circuit board and replace the board. These are relatively cheap and common at a local parts house. With a test light make sure there is current coming out the connector from the headlight switch with switch on, if not you'll need to remove the headlight switch and test for in and out voltage.

Also check for voltage into brake light switch on pedal assembly in both instances.