Are you sure the other wire is a ground wire?

The thick B+ wire goes from alt to bat to provide charging.

The thin D+ wire goes to the charging lamp / resistor then to the ignition switch ( it is on when the key is on. ) This is needed to make the alternator wake up at reasonable RPM.

Single wire alts generally need reved up to get them started, I don't use them ever and doubt Bosch makes one.

In any event, make sure the B+ spade you are trying to use is actually power out not a place to plug in an anti noise capacitor. You might have a bolt on terminal after all.

Also, if you really need to adapt the wiring, make a jumper adapter rather than cutting off the ends.