we had a 67 or 68, used to put my Peavey guitar speaker with 4 12 inch woofers in the back seat.
When I got married in ‘70 I bought a ‘67 Cutlass that the original owner had installed a nice big reverberation amp in the truck just below the back window shelf and put an eight inch oval speaker in the shelf very professionally installed. It was a killer.
It did not have suspension mounts and when the car would go over a roll back curb or big pot hole it would bottom out a component in the amp and a big boom would sound when the amp was on.