MB S211 06. Rear end i superlow both sides. I toke down the compressor and tested it with a battery and i works. Put it back but still no joy. Is there a sensonsor somewhere that might be broken?

 

There is one sensor for the rear, mounted almost in the middle of the rear swaybar, can be seen from below next to the differential housing.

When you tested the compressor, did you make sure it builds pressure all the way to max, it is very common they wear out so they only build a couple of bars.