Sadly, radio manufacturers put very little effort into AM anymore. I have had very similar problems in cars of all types. Can you get any AM reception? the AM module could just be bad. It happens.
Make sure your antenna mount is tight, especially the ground at the base. Install noise suppression caps and coils on the alternator (does radio shack even still sell those? they did in the '90s) and make sure all connections are tight.
Good luck!
(FYI, my day job includes keeping an AM station transmitter working!)

_________________
--2005 Jeep Liberty Diesel - Wife's playtoy, mine to MOD!
MAP- Cleaned!, ORM- DONE!, EHM- out the bottom with ya!
Thank you
Sir Sam