Leo...

I had the same problem and I fixed it in a rather peculiar way. I posted the answer to volvo spy. The same solution may work for you. Here is a cut'n'paste.

Liam
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I did a little investigation and I pulled out the rear c-post panel (aka sail panel). I checked the connection to the heated rear window as well as the amplifier ground, power and antennae connectors. Strangely messing about with these did not seem to produce any effect at all.

I was just about to put everything back together when i noticed that the heating element wires on the rear window (especially those parts of the wires hidden behind the c-post panel were rather dirty. As soon as I wiped them with a wet rag, I started getting better reception.

This may not be as wierd as it sounds. The previous owner of the car was a chain smoker as evidenced by the greasy black build up on just about every hidden internal surface (the dealer that sold me the car had valeted the exposed surfaces only). I am wondering whether this greasy gunk was causing some kind of a short on the AM antenna/rear heater element.

Anyhow, I guess the moral is that cleaning your rear window may significantly improve your AM reception :-)