Usually this is a seat switch problem. If you really like rubix cubes they can be disassembled and cleaned.
If it isn't the switch then it is undoutably the cable (like a speedo canble) from the motor to the gear mechanism. That is unless this is a 140 chassis. In that case it may have stripped the 1.5in dia plastic drive gear. MB will sell you a new one for around $1800... It will include the rest of the seat bottom except for the upholstery.
So far we have fixe two of these by having new gears machined.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician