The big wire has to be there already, so the other wire will go to the ring connector. It is a single small wire that is hot only when the key is turned into the START position. You will have to install a ring connector onto it and put it directly onto the starter.
The reason this was done, (the rig), was to compensate for a faulty starter or a faulty ignition switch. (A voltage drop of some sort.)
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS