Aftermarket harness.

Mattax said

IDK. They're pretty useful providing a compact, organized, and secure method of pass through.
They allow relatively easy diagnostics and isolation of individual circuits and wires.
The opposite alternative would be a cable bundle through the firewall, properly secured with insulation, yet allowing no movement which could cause were or chafing.

Starting from scratch you could do a thoughtful combination of both methods.
I've noticed that where Chrysler did use grommet feed throughs they often provided some sort of in-line wire connector nearby.

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Agreed. In my times of wiring a car myself (been a long time, back when there were no kits) I did what was easy for me, mostly point to point. Didn’t have to think about the assembly line production needs for connectors. Not that I did the best job, But it worked reliably.

My biggest mistake was not putting extra wires in the bundle. As soon as I was done I immediately figured out that I forgot stuff like wires for a tach, extra gauges, etc., and had to add them. Made a lot of extra work when it would have been no effort if I had thought about it a little more up front. There is always one more gadget you just gotta have....