The running lights in both sides work as they ahould. When the brake is pressed the driver side brake light lights up a it should, both filaments in the bulb. But on the passenger side, when the brake is pressed the whole bulb turns off. Any ideas how to fix?

 

try swapping bulbs from one side to the other.

 

try fresh bulbs, check your fuses, check the rear tail harness for damage and check the switch (looks like a little button) under the brake pedal.