I know this unit is problematic, but I was only able to find one post in the archives about intermittent FM reception (totally cuts out), though the AM reception still works, CD and tape work also. sometimes reception returns, other times it is totally out. The other post was also for my problem in the same vehicle, the 855 GLT (wagon). My antenna is built into the rear glass. I am wondering if the AM would use the same antenna. Just curious if the problem is in the head unit, perhaps a bad connection to the antenna, or a bad solder joint in the unit itself.

How do I remove the radio from the dash?

Anyone with the sedans have a similar problem?

It sounds like the antenna isnt plugged in or a good connection.
FM needs a strong signal to work compared to AM. Check the connection. Possibly if you have access to another antenna just to test. The existing cable could have been crimped somewhere along the line.
My antenna was switched to a flexible black antenna. The power antennas through my experience can be high maintenance because of freezing rain/salt/snow. It can constantly freeze and eventually the plastic track will give and snap.