Aging interior plastics are a known chore: door-pocket upholstery lifts and needs regluing (a common 124 issue documented on MB forums, doable without removing the door card), and small trim like vanity-mirror covers and latch surrounds discolor. Classic Trim Parts is repeatedly named as the go-to source for these 'unobtainium' molded pieces.
Interior plastics: door pockets, vanity mirrors, Classic Trim Parts
Mercedes-Benz W124 E-Class · cosmetic