Help fund this YouTube channel by purchasing my indepth Mercedes repair videos. Over 160 available https://mercedessource.com/store/sear... or purchase my repair kits https://mercedessource.com/store/sear... or specialty tools : https://mercedessource.com/store/sear... If you have your instrument cluster out you can test your bulbs and dimmer switch with an external 12 volt power source by knowing where to apply positive power and where to ground the instrument. Sample of a W123 engine instrument cluster is shown in this video.