Truck Forums - Help with engine wiring harness

I have a 2012 GMC Sierra 1500 5.3L V8. Unfortunately, I was attempting to go through the firewall and hit several wires and it has become a huge mess. Long story short, after mechanic bills and other issues, I have to replace the wiring harness. My question is this, all of the wiring harnesses I see only go up to the fuse box. I need the wiring that goes through the firewall and up through the dash. Is this called something different or am I just not seeing it when I am trying to find an engine wiring harness? I hope I'm explaining this correctly, if not, please feel free to comment. Thanks so much!!