OK Kane, I got some more information for you.
The pins and wire colors are correct in your diagram. The only changes are to connector "A":
A3 - 12V switched by ignition
A4 - 12V constant
A5 - 12V switched by radio, Sub Pin8
I was unable to confirm that A1 connects to speedometer in any way, but all of the other pins, colors, and potentials are correct.... including the speaker and sub connections.
Thanks for getting me started here. And if you happen to run across an under-seat drawer that can be fitted into a 451, keep me in mind.