How do I know when the rear pads on my W201 need to be replaced?
 

I've been wondering about this. There are only wear sensors on the front pads, so how will I know when the rear pads need to be replaced? Will they start squeaking? I know that some other Mercedes models have the sensors on all the pads. Why are they only on the front on the W201?

I think the first Mercedes to get sensors on all four corners was the W210 chassis.

I know that the W124's only had them on the front.

You'll know your rear pads are gone when you hear a screeching noise.

Or you can get the specs for your brakes, i.e. the minimum pad thickness, take them off and measure the thickness with a micrometer. Or actually you can get a pretty good measurement with them still on the car. On occasion the inside and outside pads will wear at different rates so measure both.

Paul, FYI our 1991 and 1992 300CE's with ASR also have rear brake pad wear sensors.

When the moon is in the seventh house
 

wait, no that's something else.

You have to get underneath and look at the pads edge on, then you can tell how much is remaining.
Everyone's driving habits are different, but I typically go through 2 sets of front pads before the rears are worn.