The car was underdesigned. It has a truely crummy alternator. The output may be 70a but it won't keep even with the fans used in A/C at idle. It can produce about 25a at idle.
The easiest real solution is to replace the alternator with one from a 90 or newer 300e. It is a straight bolt-up. You will have to change the wiring harnes but it is relatively cheap and available.
Bosch Master, ASE Master, L1
33 years MB technician