The following procedure (found in the service literature available online) details the coolant system flush for a couple of different engines.
http://mb.braingears.com/126_DISC2/program/Engine/107/M117_56/20-015.pdf

My engine being the 116.965 in an '87 420 SEL, I want to know where in the engine compartment (or otherwise) I might be able to find the "vent valve of the supplementary heater" mentioned in steps 7, 12, 16, 21 and 22 of the procedure. Does this specific vehicle/engine have a supplementary heater and associated vent valve?

Also, when the tool "flushing pipe connection with hose piece" (which I've created from fittings and hose pieces I bought at ace hardware) is attached as described in step 3 of that same procedure, should it connect between the radiator and the lower coolant hose or the radiator and upper coolant hose? Does it actually matter that much? I had imagined that it could be easier to operate if connected to the upper hose, but it also might flush better if connected to the lower hose. The photo shown in the procedure does not show enough area or detail that I can easily distinguish the location.

Any thoughts regarding the matter?