Electrical problem?
 

Had starting problem of no ignition in my 1992 500sl. Engine cranked fine. Found that voltage to ignition was low while cranking.Car does not get rum enough to keep battery up.? I think there might be something draining the battery while car sits.
I am going to install a 1.5 amp maintence charger. What is bothering me is that when car was not starting I would get it started by turning ign. switch to run position and then before cranking turning off the heater control. Is there a relay that cuts out acceseries while cranking? If there is a relay possibly it is not working. Thanks for any help

Many times on the R129 SL's the electrical portion of the ignition switch doesn't allow enough power to the EZL & then of course the engine won't start.
You are doing the right thing by adding a small charger, MB issued a bulletin on adding a charger in 1996.