CB/AM-FM adapters work but the CB performance is meh! Here's an example. I don't know if this brand is good. Used to be more common.

https://www.amazon.com/ProComm-Splitter-Stereo-Antenna-MultiPlexor/dp/B00HPH23LA

https://www.rightchannelradios.com/products/cb-am-fm-antenna-splitter

https://www.walcottradio.com/imcb-am-fm-to-cb-antenna-splitter-allows-your-stereo-antenna-to-work-with-your-cb-p-3256.html

Edit: Some advertisements say this is for using a CB antenna for AM/FM and others say it's for using an AM/FM antenna for CB. I'd never seen a couplers to use a CB antenna for AM/FM (assuming that I have now) before. Couplers for using an AM/FM fender mounted rod antenna as a CB antenna were common during the CB craze of the late 1970's. Some built-in AM/FM plus CB radios used them with a rod antenna that had a little loading coil, the same diameter of the rod antenna, in the middle. The type of rod antenna that claims to improve AM reception.

I also read this. "The inverse multiplexor is NOT designed nor intended to work with in the windshield AM/FM radio antenna. Connecting the IM-CB to an in glass AM/FM antenna may invoke the self-destruct mechanism causing the IM-CB to become totally inoperable." Here https://www.wearecb.com/procomm-im-cb-inverse-multiplexor-system.html Which fits with what I've heard about the couplers for using the AM/FM rod antenna as a CB antenna. Although "self destruct mechanism" sounds purposeful it's just unfortunate. I think something was lost in translation.

There were also disguise antennas Hustler and Turner made cowl mount rod antennas, which worked fairly well. And hidden antenna like "The Insider" made by Scarborough Laboratories and "InTenna" made by Microwave Filter Company, used the windshield or rear window opening as a slot antenna and "The Untenna" a plastic can shaped enclosure over a DDRR antenna. I mounted a few Untennas on the underside of vehicles for low band VHF public service radio reasons. Worked better than expected but not great in that location.

CB theft was becoming a big problem some mobile radios cost hundreds dollars... in late 1970's dollars. Hiding/disguising the antenna and quick/slide mounts for the radio helped a little.