Wiring help

[timski69gmc]

Good day. My friend helped with an engine swap last month. While the engine was out, we painted under the hood. So everything got disconnected.
Most of it makes sense. But I have a loom coming out of the firewall above the main firebox harness, with a heavy red, a black, and a bundle of a blue, green/white and a brown/white with a connector.
I have no idea where this came from. Have a wiring diagram but can’t find this combination.
Any guidance would be appreciated.