I havn't done the rear brakes on my 98 E320 but have done the front brakes.
Looking at the rear brakes I also have only one sensor on the right side - no wear sensors on the left.
Look at the rear rotor - the minimum thickness is usually stamped on the rotor.
Keep in mind that the minimum rotor thickness is based after turning the rotors.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles