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Hi Paul, it sounds like they soaked the electronic selector module and damaged it. The module is the complete gearshift assembly.
You will need to get the car to a Mercedes specialist to read out the fault codes in the transmission control module. More than likely there will be communication faults stored with the selector module.
Sometimes erasing the fault codes will fix it but if it doesn't work they will need to replace the shifter. Another possibility would be a bad speed sensor in the transmission but that would have nothing to do with the detail. The fault codes will point to the problem.