This is a simple diagram of the original MGA wiper circuit. The one green wire that powers the wiper motor will be disconnected from the fuse terminal, and two blue wires on the new control unit will be hooked in series between the fused power and this disconnected wire. You could run the two new wires through the bulkhead and make those connections right at the fuse block, but I have a slightly more devious method that hides the new wires. It is notable here that one of the green wires from the fuse block A4 terminal runs to the back of the fuel gauge where it may be convenient to pick up this "switched and fused" power for the new wiper control.

At the bottom of this image is a small diagram depicting the internal connections of the MGA wiper motor, showing how the rotating park switch maintains the ground connection to keep the wipers running to the park position after the wiper switch is turned off.

One of the nifty functions of this new control unit is to also maintain the power connection after the ignition switch is turned off, so that the wipers will continue to run to the park position and not be stranded in mid wipe when the key is switched off. That prevents the wiper motor from trying to run when the key is switched on again later (possibly much later) when the wiper blades may be stuck to the glass, which could result in burn out of the wiper motor. The new control unit also defeats power if the wiper switch is left on when you shut down and leave the car, so that the wipers will not start up again until you turn the wiper switch off (for a couple of seconds). This is another way the new control can prevent motor burnout if the wiper blades are stuck to the glass. The wiper motor will not try to run again on restart even if you have left the wiper switch turned on.