How to tighten a rear view mirror for 97 Mercedes Benz S320?

You will have to remove the inner cover that will give you a better idea of what is loose. You will have to make a wooden wedge to do this or use some other material to pop the inner cover off at the top and then pull it upwards. There are some retaingin screws that you will see if you need to pull the mirror assembly off. However, you should probably leave the cables that adjust the mirror intact and in place as it is very difficult to reseat them properly. This may give you a view into whether a screw is loose that can be tightened or if the teflon ball that the mirror pivots on needs replacement. You may find that one of the cables is loose in its holder. You will have to replace the mirror assembly if this is the case.
I inluded a diagram of the inner panel to remove with a wedge to get a look at the inner workings of the mirorr. This should be adequate too see if any of the above are the case without having to remove the mirror assembly.