Here's some interesting tips on Broadcasting AM in stereo and still enabling a fair signal for standard car radios, excerpted from:
AM radio high fidelity Part 15 broadcast
December 24, 1999
https://www.scivision.dev/am-radio-part-15-high-fidelity/
Selecting AM Part 15 license free broadcast frequency: recent encroachment by second adjacents means at night calls for audio transmit bandwidth 10-15 kHz.
Non-CQUAM car radios might have only 3 kHz of bandwidth. Center frequency is 1640 kHz has modulation from 1620-1660 kHz at day, and about 1630-1650 kHz at night.
To get the best frequency look two channels (20 kHz) up and down from the intended frequency. If a third adjacent is nearly (30 kHz) go an extra channel away from them for poor selectivity receivers.
At night it’s tricky to find a full bandwidth clear channel, even in the expanded band, which is the only place for license free AM that gets decent range day and night. ... ..
Note! Unless running less than 3-4 kHz audio bandwidth don’t use 1700 kHz center frequency since the cutoff for Part 15.219 operations is 1705 kHz–that’s absolute, not center frequency of 1705 kHz.
Another trick is for the receiver to deliberately tune off frequency, such that the carrier and one sideband are captured. This only works for analog receivers of course, and the trend is to digital frequency receivers. ....... ...
Improving Part 15 AM broadcast SNR and range: at nighttime, audio compression would help make apparent SNR higher by increasing the loudness of quiet passages. Adding an expander circuit to the receivers akin to Dolby B noise reduction would add nearly 10 dB apparent SNR. Keep modulation clean and near 100% peak to maximize coverage. .... ... (remember licensing for public performance issues)
Where SNR is adequate and with receivers modified to pass 20 kHz, a Part 15 AM MW broadcast system can exceed the fidelity of FM radio.