General Motors offered AM stereo on several of their car lines back in the eighties, Cadillac for sure... I acquired one, replaced the cassette belts, and put it in my Olds Cutlass real wheel drive. I never actually used it, kind of novelty, used the cassette, but there were a number of Southern California AM stations broadcasting in stereo and it definitely had more fidelity than regular AM. I forget now but the technology made use of both the upper and lower part of the carrier which effectively doubled the bandwidth and increased the frequency response to 10K, which is a great improvement. As an aside, that GM unit was a performer... had music search, auto reverse utilizing a four channel head, five band equalizer, frequency derived motor speed control, four bridged output amplifier stages, switchable dynamic noise elimination... a well designed unit with excellent build quality.