The reason for custom building the harness is ease of install. Here’s the route of the two chassis harnesses you ordered, so everything you see in red needs to be removed and replaced with the new ones-
It looks easy on paper, but once you get under there, you’re gonna be reaching up around all kinds of obstructions and trying to pull wire loom zip tie trees out of body holes, then fishing the new harnesses in.
The module is at the passenger side of the glovebox area (I believe there’s a detailed pic earlier in the thread).
If the connector for it isn’t there, then you also need a different instrument panel harness, which requires dash removal to replace it.
@johanh13 is building a stand alone custom harness that doesn’t require removal of any other factory harnesses, and it is using the exact same connectors and terminals as the factory uses, and will be connected to the hard parts just like factory. I believe he paid ~$70 for all materials to build the harness, whereas the two chassis harnesses alone are about $380, and that hasn’t even touched on whether or not you need to swap the instrument panel harness.