Drive to the bus-stop "Steineck Chapel" (german Steineck Kirche) in the south of the map and stop there. Look at my screenshot where you have to walk now. The chapel is on a little hill and there are the two people, who just got married. Go towards them and it will unlock.

Look here at the map in the south:

If my guide helped you, please leave a thumps up for me, that would be nice. Thank you.