I just installed a new kit from zex and did the same thing. I burnt my line because the pos amp wire fell and hit the nitrous line in the trunk and it cooked like a two foot section. and the wire you want is the middle one of the three, at least it is on my b16. if you are un shure just take a multy meeter and set it to the batt setting and test each wire one by one. wich ever wire goes progressivly higher in voltage as you pull the throttle open with the key on is the wire you want. to avoid making a mess of your wires just to test them, use a pin to stick through the insulation insted of cutting it.

 

or like a sowing pin those work really well...