I've been told, that the backup switch extension harness (FM12695) plugs into the dash harness and comes out of the firewall to the outside of the brake master cylinder. It then runs across to the passenger side through the wiring gutter and drops down to the backup switch harness (FM12690) on the side cover of the transmission.

I found a GM neutral safety switch bypass for manual transmission at Lectric Limited, https://www.lectriclimited.com/neutral-safety-switch-bypass-harness-99050#