blk/red is the antenna trigger. are the factory plugs still on it?
you should have 4 twisted pairs of wires. these are front and rear. the gtr has that annoying amp in the rear shelf that should have been bypassed already.
red/blu is lights , should be a blue/? one that is +12v key and the other one should be a red..
( haven't worked on a skyline for a few weeks so I can't remember the colours.)
if the plugs are still on it buy an adapting plug and use that. makes life easier.