Which Washer Bottle

There's a screenwasher switch attached to the back of the wiper switch on cars so equipped. Pressing the wiper switch knob activates the washer switch, as the wiper switch shaft extends through the length of the switch body so it can press the washer switch. If you can't come up with one of the washer switches or an appropriate wiper switch with washer switch already attached, it shouldn't be too hard to rig it up with an off-the-shelf pushbutton momentary-contact switch.