When turning on the windshield wipers or headlights the dash lights turn off?

My mother just drove cross country and says when she turns on her windshield wipers her dash lights shut off about 3 seconds later. If she turns on her headlights the dash lights, go out immediately. We have checked that her dimmer switch is on the highest setting. She also says, if she turns the interior lights on, and sets them too high, then the wipers and headlights do not affect the dash lights. We are wondering if this is a fuse issue or an electrical short. She will be continuing her trip in a few days and is concerned about the potential for fire if it is electrical.