Hey spin. First thing first. Clean your connections
put a votl meter across your terminals. A fully charged batt not running should be a min of 12.6 volts. If that looks ok and headlamp turn on. Then you have a Circuit #30 problem which is main circuit from batt to igintion switch.
Spin do this right by the brake master cyl against the firewall a 3 pin connector black in color. Pull it off
the middle pin is circuit 50 Stater circuit female side
put a test light in it turn key over like your starting it.
If it lights up your igniton electrical portion is ok. If not then you have a bad eletrical portion of the switch.
Next thing the male side if the connector purple lead wire going to starter. If you have a remote starter switch
put one connector at the middle pin the other to batt positive try to start it. If it cranks you elimnated the circuit #50 to the start hope this helps.
Most likely I feel from my experince as a Factory tech 14 yrs. Its going to be the igniton switch .
Hope this help spin.
Guido Factory MB tech 14yrs
1993 500E 85K pearl black