The cross over box mounts to the top of the glove box liner. For some reason I can't think of the 70 rear speaker switch location exactly. I am thinking it is under the dash near the right knee of the driver. Hopefully someone has a photo.

70 Road Runner 383-4 4 speed FJ5 & black guts
70 Charger R/T 440-4 4 speed FJ5 & white guts