how do you adjust the trunk close piston

If your car has an electronic closing system for the trunk one of the struts that assists the motor, or the motor it's self could be bad. If the trunk is manually operated, the springs that assist in opening may be adjusted incorrectly or the struts could be bad. I recommend having a Mercedes-Benz dealership take a look at it. Here is a link to help you find one: http://repairpal.com/directory