i'm not using my truck ECU, just the LS1 ECU. on the passenger side plug i cut all the wires that ran to the truck ECU. i keep them & just tucked them into the harness. most of those wires i think are just for data, but there's a few that run the blower motor & some lights. my headlights & dash lights weren't working till i plug the 2 plugs back in under the dash on the passenger side.

that plug, actually a weatherproof grommet, is the only thing i couldn't find an actual wiring diagram for. i can find a few diagrams that show some of the wires that run thru that plug but not all of them.

i'm sure you could maybe make a smaller hole & reroute just the one's you need, but you definitely need some of them.