I can believe AM radio reception is no longer as good as it once was; in general and not just car radios. There are several factors that contribute to this, I think. Some factors, like the radio quality and antenna design, are within your control. Others, like transmitter quality/power and antenna design/location are not. You may find more thorough discussion on "audiophile" or "antique radio" web-sites.
If looking for something modern doing the best that can be with AM, I think you will have to be looking at high-dollar stuff. Ignore all the FM features/frills, just figure it would make little sense to bother putting in poor circuitry for just the AM, rather than use existing (good) FM circuitry for both bands. And, as mentioned, a good antenna may not look even close to stock; may mean punching a hole somewhere in the body.
A local car-radio shop may be helpful; but I doubt it. Those places, doing aftermarket radio/alarm/RKE/lighting, seem to me to be of generally low quality. And AM is largely an orphan now.