Thank you for your reply and the information.
Looks like I need to do some experimenting.

Would a screen washer pump provide enough pressure to force water in to a sealed pressure tank? Somehow I doubt the theory.

Id try it out in the bath first before putting anywhere near a sub.
This way you wont need to worry about the switch wiring till the ballast system works properly.

You may need to fit a geared pump motor to get over the pressure effect, or it will simply flip the rubber paddles inside the pump element and stop the pump from working once it reaches a certain pressure, same happens if a washer system blocks on a car.