What do you hope to achieve by adding a relay?
How will it improve the operation of the wiper?
A relay is a switch - a switch that is controlled by an electric current.
There are two common uses for relays in cars, the first is to allow a computer to switch a circuit on or off - for example the fuel pump - and the second is to allow a low current switch to control a high current load - for example, the horn or fog lights.
There are three yellow/blue wires in that diagram, that "disappear" off to the left so we have no idea what they are connected to, or even if they connect to one another - my guess (based on the diagram for my vehicle) is that they do supply power to the wiper system, and that the one connected to the motor is only used to "park" the wipers when it is switched off.
I don't see how adding a relay/switch to that wire would improve the operation of the wiper in any way, I could however see it making things worse - like the wipers no longer parking automatically or maybe not even working at all.