What's this AM you speaketh of... 
Actually, your issue is likely the antenna ground in your passenger side A-pillar. If you remove the trim (remove the visor and pry the trim away - making sure you release the hiddne clip at the bottom w/ a long screw-driver) you'll see a black plastic triangular tube ~ 3" long. Remove the screw connecting the wire from it to the A-pillar. Use some sandpaper and sand off the paint and primer until you have just metal. Then you can put a little dielectric grease on the spot or just refasten the screw/wire. I'd test the radio before you reinstall the trim to make sure you sanded enough off. CAUTION: when reinstalling the trim and visor, DO NOT over-tighten the the two outboard Torx (T-30 ? IIRC) screws
. Just barely hand tight is fine. Don't ask how I know... though I wrote up a solution for that, it's best if you do not need it
There are probably directions w/ pictures in here somewhere if you search on AM or radio reception...