I use a ClassicParts.com harness (12-volt generator conversion) all my wires run from the main harness you see on the right that is how they set theirs up.
https://www.flickr.com/photos/advance-design_parts_co/8129206782/in/set-72157631867396207
I am not a big fan of seeing wires under the dash, so you can see everything is up and out of the way.
https://www.flickr.com/photos/advance-design_parts_co/8129204902/in/set-72157631867396207
To wire my hidden stereo I even snaked the harness up the windshield post over the door and with a die-ground hole through the mid-body break down behind the seat. I spent a lot of time on wiring and it has paid off as I have no issues with it.
https://www.flickr.com/photos/advance-design_parts_co/8129072768/in/set-72157631867396207

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr