The thermostat is located on top of the waterpump under the thermostat housing (usually metal) If it has the black plastic housing you should replace it with the metal style when you do the thermostat, because it will definently break off at some time leaving you stranded! There are just 3 10mm size bolts that hold on the housing to access the thermostat. If you remove you radiator cap (when cold) squeeze the upper raiator hose and while holding pressure on hose, put cap back on to create negative pressure on coolant system. This way you will have very little coolant loss. Install new thermostat and top off coolant to proper level. Thats it!
Factory Trained Mercedes-Benz Technician