When you want to clean the window on the backside, you can wipe it and give some water to it. It takes some time for the water arrives, I solved this issue by mounting a one-way valve in the hose.

On this little valve there is a little arrow dat marks the directions it should be mounted.

This little valve replaces the black connector between the 2 hoses on the picture below.

But first you have to remove the plate above the reservoir.

On the left side there is a plastic screw you have to remove.

The shape of the screw can be seen on the picture below:

The cover can now be removed:

Now you can remove the black connector between the two hoses and replace it with the little valve.

Watch the direction of the little arrow on the valve! It works only to one way.

Just mount the two hoses firmly to the little valve. The cover can be mounted again  and dont forget the plastic screw!