This is a replacement for the rain sensor on the mower. Assembly is easy, drop in a 30x30 fan on the top, route the wire through the hole and place te cap (deksel) on top (clicks on) The sensor can be removed by prying the clips from the inside out (careful, the plastic is weak). Then drop in the duct with the wire for the fan routed via the inside.

For the openmower board: connect the fan onto the M1 connector (it's near the pi connector next to D1).