There is not one simple answer. There are a multitude of reasons your trunk closing assist does not work. In the MB S500 the doors are close assisted and they operate on the same system as the trunk. Do the door close assists work and the trunk not? Broad questions get broad answers. You can go to the related link to learn much about how the closing assist works on the S series of MB.
doors locks & trunk lock not working
with a crow bar. If the inside of the trunk can be accessed by removing the back seat, I would try that first.
I have a 1987 Mercedes that has no trunk key. Do I remove the back seat to get in?
The convenience relay that is located in the trunk of a Mercedes C280 may need to be replaced if the power seats are not working. Bad or loose wiring can be another cause for power seats to stop working.
Make and/or model?
All Mercedes have battery in the trunk
In the trunk
How do you replace trunk lid springs on a 107 body 1983 mercedes 380sl?
push a button
How can you open the trunk to your mercedes benz car?
I think in trunk right wall, inside the trunk liner.
drill the lock and push in, lift open the trunk door