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.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)