READ VERY CLOSELY: I fully understand a malfunctioning dimmer switch will cause both sets of lights to cease working. I also understand that it is about the easiest electronic component in the car to bypass to validate its functionality. If you can’t figure out how to bypass that switch before blindly buying and installing a new one, you have no business troubleshooting this or any other wiring issue. I personally gave you the benefit of the doubt because few of us have boxes of switches lying around to take the “Install it first and see what happens approach.” Many parts places refuse to take back electric components, so I will never recommend replacing before testing. The beauty is he can listen to one, all or none of us, the choice is his.