On my Busa I use a Dyna Shift counter, loop the 2-3-4-& 5 gear post together and run the low 12v wire from the nitrous relay from the 2 gear post.
I use the shift counter because I can have the nitrous come on in any gear I want not just in 2nd and stay on.
Along with the shift counter I use Greggs nitrous harness so the ECU can turn my nitrous on @ 6,000 & off @ 10,8000 - with the TPS 85%
You can get one on E-Bay pretty cheap