Ditto Dukey.

I took a ferry from Ancona and drove the car on board so we could drive across Greece and beyond.

Thanks everyone! You've all be very helpful, but here is the dilemma: It's hard to believe, but to rent the car for a month and drop it off back in Germany, it will only cost $1300, including taxes and GPS. If we drop the car in Ancona after using it for 12 days, it will cost us $4500!

If we take it on the ferry, we'll have the ferry cost, plus $8/day surcharge for going into Croatia, plus parking in Split. (We're still considering this option which will place us in Croatia on the 13th with 4 days between Split and Frankfurt.) (In which case, more Croatia and Austria, but no more Italy!) Capice?

Do keep in mind, with all the suggestions, that the only cars parked trunk-backwards against the wall in the parking garage in Ancona in the exact same spot and position for more than a couple days will be the ones with "stuff" in the trunk...
whether it is valuable or not, will be for any thieves to decide during the week they have to observe and eventually act in the heart of the night...

There are also no longer any luggage lockers in Italian train stations (security worries) but the Ancona train station has a staffed left luggage office which is probably just as, if not more secure.