The jpoc guide to washing your car in Germany.
Yes, just as with everything else, there are rules about washing your car in Germany. Originally these rules were introduced to reduce the amount of soap that was making its way into the waste water system. Now, there are additional goals: reducing the amount of oil etc introduced into the waste water system and reducing the overall use of water.
The upshot of all of this is that you are only allowed to wash your car in a designated place and such places must have the facility to remove oil and detergents from the water and also to recycle water.
Normally, these places are called Waschstrassen and they vary from a traditional mechanised car wash system to a place where you may wash your car yourself by hand. Whichever you choose, you have to pay and there are restrictions on the hours of operation. Mostly that is to reduce the disturbance to others.
You can be sure that even a small town will have at least one such place that you may use. The easiest way to find it is of course to ask your friends or co-workers. Everyone with a car in Germany has to use these facilities and so somebody will know where you can go.