I have a permanently-mounted smart charger on my '96 E300D; the cord comes out down behind the removable panel in the lower bumper surround along with the block heater cord. All I have to do is to plug it into an outlet any time I want to top up the battery. Has been on the car for several years without problems.
One thing to watch is that all of these "smart" chargers draw a little current. It isn't enough to harm the battery but if the car is put away for a significant portion of time (all winter, for example) than the charger needs to be either disconnected or connected to power, which you would probably do anyway.
If you are at all concerned about over-charging the battery, just put the charger on a timer so it only works a couple of hours a day.
"Buster" in the '95
Our all-biodiesel family
1995 E300D (W124) 201,000 miles My car
1996 E300D (W210) 289,000 miles Wife's car
"Politicians are the same all over the world;
we build bridges where there aren't any rivers."
-- Nikita S. Khrushchev