i don't really get what you are asking when yuo say the best palce to hook up the ITPS ... but you want it to be between your battery and your pw-70a. grab a constant 12v line (either from the battery itself or from something else in the car that has a big enough fuse) and a switched 12v from something else. that's it.
is the original wiring harness still in the car? if so just buy a harness from best buy and it will map everything for you. if you cut the harness out ... shame on you harness cutters deserve what they have coming to them