Order a small switch from your favorite electronics parts supplier:
I live in the Netherlands, and I ordered it from Conrad, the order number is 701074 – 89, the switch only costs 1.01 euros. I ordered 3, you’ll never know when a small switch like that will come in handy.
Solder two wires to it (and use thinner wires than me, because the pins of the switch are really small), and drill the hole where the valve was mounted a little bigger so the switch will fit (depends on the switch you use). Put the switch in and use little spacers to determine the right distance:
Now wire the switch so the windshield washer motor is activated through a relay. The switch I used is 125 V~ / 0,5 A and I don’t know how this will behave at 12 volts and a lot more amps which you’ll have in a car. So you MUST use a relay!