Carl,
Guessing one of the wires is black, and the other maybe blue? (Looking at a wiring diagram from an 06 Sti downloaded from the manuals section of the forum, but its USD)
Depends on how MMS wiried it up, (from the diagram) these wires from the motor should go back to a relay, two of these wires, one goes to ground, the other to to water tank low level switch so the pump won't run when the tank is empty.
Back to the two wires you can see, providing ones black, the other colour should go to the pressure switch in the engine bay. If you know where the pressure switch is for that, its simply a matter of running your "manual" switch in parallel so you bypass the pressure switch, or run a 12 volt supply through your manual switch to the pump motor (coloured wire).
Any idea how long it takes for the pump to empty the bottle? or how much water is used during your longest run? Just asking so you don't empty the bottle before you do your run.
