Big 3 upgrade

[nkd90]

Im looking for information to do the big 3 upgrade on my 2015 SLT sierra. I know how to do the big 3 but on these trucks I cant find the negative lead from the alternator to the chasis. From all I can see is one postivie wire coming from the alternator and the only other thing connecter to the alternator is a pluggin at the top with like 10 wires or so but nothing else.

also if this is gonna be almost impossible what battery would someone recommend for just running a dual battery setup. I know how to run batteries in parallel just unsure on how many cracking amps are needed for this truck.. I couldnt find anything on the web! Im trying to not spend 600 on two batteries so I was just wondering what the manufacture cranking amps was for the truck so i could buy batteries that are within the range because buying a 1200 CCA is not needed or reasonable where I live!