No "AM" station in my 1998 Jeep grand Cherokee.

Car drives perfectly even at night, no warning lamps in cluster?
are we to guess?
is FM is ok, ? if not then the antenna is bad,disconnected or missing.
do other modes, work, like CD? CDDA audio disk work.
is radio stock, we cant guess that either. photos posted a win.
do you hear noise? (called static, ) if yes, then the amp is good.
but if no noise, then turn up volume, got noise now?

if FM works then my guess, AM stations are too far away
where are you, or where is your nearest station.
try mega stations, 50,000 watt super stations. hear those.?
like KGO out of SAN FRAN
does the AM work at night, ?

try one of these near you, for AM.

http://www.northpine.com/broadcast/50kwam.html

if not in USA< say so.
I have the lists world wide. (ARRL.COM)
I repair radios.. so.....

Press the search or mode button to get different band widths
consider that there may be no am stations in your area