Waiting for someone to mention 4-tracks. My dad bought a brand new 1967 Mercury Cougar with the latest 4-track sound system, and he wasn’t too happy when 8-tracks took over within a year.
My cousin had a 45 record deck under his dash. I believe it was a Sparkomatic, but I could be wrong on that. You put in one record at a time and it had some sort of pressure device that pressed on the needle to keep it from skipping when the car went over bumps. It didn’t work very well, and sounded like s#it but I have never seen another one.