I'm not familiar with the 98 model. Does it have wear sensors on the rear? Mine don't, and even if you do they are not foolproof. What do the rotor surfaces look like? Brake pad wear light comes on at 2.5 mm thickness. Have you checked the thickness of all four rear pads? If they check okay you may need to remove them and check for grit. Sometimes small pebbles can get lodged somewhere on the wear surface.
95 E320 Cabriolet, 131K