This snowball is rolling downhill on the '73 Mustang build. I was ready to start dropping the wiring and electrical components into the car and had Dad pull the rear seat to provide room for routing the harness. He texted me a picture of the wonderful hole he found in the rear seat pan.

It wasn't visible from beneath the car as the hole was/is directly above the framerail/spring mount.
We stripped the interior out to find that the whole driverside floor is going to require replacement. There are holes in several places that were hidden by the frame or undercoating.
Along the way, we found that Ford had done a real bang-up job of routing the rear chassis harness; in fact, the car nearly caught fire some time in the past due to the dome light wiring shorting out on a sharp edge in the driverside C-pillar. Both the edge of the sail panel and the edge of the rear seat are slightly charred.
So yeah, this snowball is rolling down the hill.

We did find the build sheet under the sound deadening on the driverside rear footwell. Unfortunately, it's trashed.