Juat a few random thoughts on this ...

1. You might consider upgrading to an aftermarket radio that has "HD radio" capabilities? While HD Radio is primarily advertised as a benefit to FM listeners ("digital, near CD quality sound" and "ability to tune in as many as 3 additional sub-carrier stations broadcast along with the primary station's content"), many AM station also support HD radio, giving you FM stereo type sound quality.

2. I would think you probably could upgrade the antenna on the car to improve your AM reception, but that would probably take some experimenting with various lengths of antennas and so forth. The last time I checked, most car stereos simply used the same antenna connection for both the AM and the FM reception. (Some may simply use an internal coil type antenna in the radio itself for AM?) The ones that work best have diversity antennas from the factory, but you rarely see these anymore except on a few high-end luxury cars. (The diversity setup usually has one grid-type antenna built into the window glass that's optimized for AM reception, and a second antenna connector from the stereo goes to a standard whip or telescoping antenna for the FM.)