It's easy once you get your head round it Si
A multimeter will be handy so you can figure out which is + and -
I know the GTR pumps have a 2 stage voltage thing, but I presume it's ECU based rather than pump based which is why a GTR pump sounds loud in an S-body, cos it gets full voltage all the time.
Either way, getting a good feed direct from the battery and using the stock + wire to trigger the relay will give a guaranteed good strong power feed that is still ECU controlled for priming and such