well actually there is only one place to be:
Stuttgart - you can get there via lufthansa directly
should be no problem. Main factory is in Untertürkheim
but also in Böblingen, you should not forget the new
Mercedes museum, which is really worth visiting!!
Morover
porsche is just a short taxiride then and
also worth visiting, however you will have a hard time
to reserve a tour, since they are booked for months
hope helped you a bit...