Water/meth injection must be between the MAF sensor and throttle body. The MAF sensor is calibrated based on the specific heat of air. Mixing anything else in with the air throws the calibration off.

Yes you’ll want it between your MAF and throttle body. You’ll need you intake air temperature sensor (IAT) to also be in the intake tube to pick up the change in the air temperature when adding the meth. This isn’t my exact car but it’s VERY close. I have a Procharger on my car and I had their plastic connector screwed into my intake which then my IAT locked into that. Long story short my IAT sensor wasn’t far enough into my intake tube that it wasn’t picking up the cooler temps with the meth. So make sure you IAT sensor is expose into the intake tube.

This appears to be a nitrous only application, no forced induction. Not sure IAT is still important. With the plate for nitrous, the intake charge is going to be cooled further as it enters the manifold.