Is this the pre-facelift W211 Sedan model if it has the GPS antenna at the top of the rear window (the facelift has it on the roof)? These are prone to fail but you could check the connector at the far front/left/top, near the hinge at the trunk.
You should also set the COMAND to show how many satellites it can detect. If it shows "-" the connector is unplugged or the cable/antenna is broken. If it shows "0" when it should not, it must be the antenna.
You should also set the COMAND to show how many satellites it can detect. If it shows "-" the connector is unplugged or the cable/antenna is broken. If it shows "0" when it should not, it must be the antenna.