Could also be your antenna lead wire, or speaker as well. AM can sound great.
A lot of it depends both on the stations, and what they are broadcasting. Where I live, we have a local world music station, that always sounds muddy. Driving around, I've noticed that most stations that are playing "newer" music sound muddy on my AM radio. We also get an oldies station (KIXI), which sounds just great, much clearer. I also find the same thing when I play music though the aux input, big band and Perry Como sound great, newer stuff not so great.
