The AM and FM parts of your head unit use the same antenna - but AM is way more fussy about the antenna. You say AM "does not work", unfortunately, this tells me too little to help you, so let's go on the assumption that your FM does work and your AM does not work at all: the problem is in the head unit. Buy a new one, go on eBay and get a used OEM one, etc.

Now, if what you mean is that FM sounds fine, but AM is full of static, pops, scratches, noise, exhibits very poor range, etc. - then I'd guess you have a poor ground connection on the antenna, a bad antenna whip itself or bad "suppression ground(s)" or "bonding ground(s)" somewhere in the car. You might also have a poor connection between the shield of the antenna cable and the connector at the head unit end of the antenna cable. Again, AM is way fussy-er about the antenna system than FM.


Mark
(Radio Systems Deployment)
Kenwood USA Corporation
Systems Division - Atlanta