Back in 1971 while I was still in the USAF I owned a 1967 Pontiac Le Mans that had the standard AM radio with one dash speaker and two rear deck speakers. The Base Exchange got some car AM/FM/FM Stereo units in stock and I picked one up. It came with several trim plates so installation was fairly straight forward and looked great installed. FM stereo in cars was still relatively new back then, so I was anxious to hear what difference it would make. I decided to make the front speaker the left and the rears the right which worked quite well. My 'moment' came with the first FM stereo rock station I tuned in that illuminated the red 'Stereo' indicator on the dial. Glorious stereo music filled the interior and I was blown away with how great it sounded. I went on to include an 8 track player into the system and to this day I still have fond memories of that car and the many road trips I made with it while listening to my favorite tunes.
BTW, here is an interesting timeline concerning automotive sound systems.