German law enforcement says the dumping happened Saturday in a town about 18 miles north of Munich, where the 52-year-old dad and his kid were parked roadside ... minding their own business.

That's when a tractor pulling a trailer full of manure swung in their direction while driving by ... and unloaded the crap on them and their Renault.

There ain't enough steam cleaners in the world to save the car. Police said no one was injured -- but that's easy for him to say. The father and daughter were reportedly covered from head to toe.

Just like Marty McFly would draw it up.