Car Rental STR
 

I need to rent a car at Stuttgart airport to drive to Konstanz. My flight arrives Sunday night at 23:30.
Does any car rental in STR have safes to get the key after the station is closed?

Rentals usually stay open if someone has made a booking, alternatively just call the station and ask if they will wait for you.

Alternatively, you might rent a car at Stuttgart Central Station (ZWS) where they definitely have safes for the keys (Sixt & Europcar; they used to be near plaform 15/16 but may have moved them as those two tracks are now closed due to construction) - however, I'm not sure whether there is still an S-Bahn connection to ZWS that late in the evening. The last connection from ZWS to STR leaves around 23:25 if I remember correctly.