I have the same problem with intermittent FM in my 850 wagon. The wagons have the antenna on the left rear glass, I followed the wires from the antenna and discovered that there is a little gizmo intalled behind the interior panel. It has connections from the antenna on the glass, the coax to the radio, and two other wires from the electrical harness. It seems to be some sort of switch or amplifier.

I determined that the radio is ok by grabbing a long piece of wire from my junk box and touching it to the center pin on the coax to the radio, I got FM reception. Plugging the antenna coax lead back in and disconnecting the two wires from the wiring harness made AM go away. So at this point I suspect that the amplifier / switch box is the source of the problem, and I'm off to find a replacement and/or take the existing one apart to see what I find.

Removing the interior panel required TORX drivers - Sears has a nice set if you don't already own them.