Radio works on FM and for no identifiable reason, the AM reception dropped to poor

I figured it out. The radio uses the rear window defroster as the AM antenna. There is an Antenna Module above the rear seat behind the plastic cover that supplies power to the defroster AND has a slot for the AM Antenna cable. I replaced the Antenna Module AND then had to go to a junkyard for the power cable which melted the input cable slot and caused the problem. Turns out this is a Park Ave known issue since the cables I found in an older junked car HAD THE SAME ISSUE!

I know that the 2000 Buick LeSabre used two antennas for the factory radio. The FM used both the front windshield and also used the defroster strip in the back window for antennas. Both were coupled together with a powered module to provide diversity reception on FM. The AM section used one of the antennas only. If you install an aftermarket replacement make sure that the module gets power or little or no signal will reach the radio.