Red light in door not red

Does anyone know how to access the red light in the rear drivers door to change the bulb?

Do you mean the one that is on the door edge to warn other cars your door is open?

If yes - pull it out, or use a wide flat blade screwdriver to gently prize it out. Simple sprung plastic clip that is part of the lens and is along the short end of the light.
They also automatically switch off after ~10mins or so when you leave the door open so bear this in mind once you've changed the bulb!