The Audiovox FMC-1C is a well-regarded FM converter from the late 1970s. It can be picked up at yard sales, but beware the possibility of a dead unit. The basic principle of operation is that the FM broadcast 88-108 MHz front end downconverts to 10.7 MHz IF, then into a TA7130P IF amplifier / detector IC, that is then modulated to 1.400 MHz AM signal. Think of it like an FM broadcast receiver, and AM broadcast “transmitter” all in one box. The AM-only care radio is tuned to 1400 kHz and then tune the FM converter to the desired FM frequency. Even if the AM radio is AM stereo, the FM signal will be mono, as I don’t know of any FM-AM broadcast converters that have an AM stereo generator. The music fidelity is primarily limited by the IF bandwidth of the AM car radio, perhaps 3-6 kHz or so.