From the PWI-1 pdf
Locate Green 20awg wire and tap this to the negative side of any fuel injector. This wire is used to monitor injector pulse width, and is very important to solder this connection. If you are unsure about which wire is the negative injector wire, use factory service manual wiring diagrams. Note: This wire can also hook to a specified input on aftermarket ECU's to allow for full 3D mapping of water injection.
What input does the take? I am getting conflicting answers from aquamist and Perrin. Really want to buy one, but I need to know what I can hook up to it first.