Mounting Fader Switch for Rear Speaker

Best I can recall it mounts on the lower edge of the dash 3-4 inches to the left of the knee-knocker A/C registers. (Memory is fuzzy here, parts manual not a help either.)

You're right, there was only one speaker in the rear with the factory AM/FM. It mounted in the passenger's side cut-out in the package shelf in the rear.

FYI: The AM radio was rated at 2 watts (peak), but the AM/FM was rated at 5½ watts. No stereo radios offered from the factory for the A-bomb in 1972.