Finding any info on FM stereo car radios (vs. mono) is very hard. I found one post that indicates 1967 Toronados (and I would assume Cadillacs) had a radio with a multiplex jack. An optional stereo adaptor setup could be ordered. This doesn't make it clear if the main radio or the adapter were tube or solid state.
https://forums.aaca.org/topic/40361-...io-in-any-car/