[Gran Turismo]

Check out "touchless" car washes in your area featuring undercarriage water jets as well.

Edit: Since you're in Germany, also check for "Textilwaschanlagen" offering an "Unterboden" program. If operated properly, textile brushes won't harm your paint. A good indicator for the latter is the amount of nice and expensive cars (obviously owned by car afficionados) queueing up at such a place.