Have you tried a flush and fill kit by Prestone?
You should be able to tap into the heater hose
connection with the tee. The tee is available
separately, and it has a garden hose connection
under the cap. You put in what ever cleaner
in the cooling system after removing the thermostat and run car for at least fifteen minutes. MBZ has some kind of cleaner. It has
been posted before. Should be in search. The
traditional way the flush and fill kit works is
to exit out of the radiator opening. You could
remove the hose from thethermostat housing and allow that as your exit.
The hose screws into the tee forces water
out of the thermostat housing (remember you
have removed thermostat and have put cover back on
previously) to allow maximum out flow
of water and gunk.) Be sure to use a neutralizer
if you have any type of acid flush cleaner.
Hope this helps,