follow the upper radiator hose (the big fat one) to the front of the engine - it will be attached to the thermostat housing. three bolts to take the housing off. when you pick up the new thermostat - check that you have a new gasket included because the old one will be shot...
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"