I think the w124's have a dash lamp for a bulb out and this light will illuminate if you replace your regular bulbs with LED's, in addition to this if you replace your turn signals they will blink very quickly due to the voltage drop (LED's consume less power than filament bulbs)....Also the quality of light will be poor due to the tail light housing (reflector) in the w124s... I think there is a vendor on ebay that sells LED Tail Lamp housings..... I bought a set of LED's and the quality of light was horrible, could barely tell they were on.