One problem related to rear window defogger has been reported for the 1999 Buick Lesabre. The most recently reported issues are listed below.
The event leading to the incident involved turning on the rear defroster. After turning on the rear defroster I smelt smoke and burnt electronics. The rear window would not defrost. I turned the car off and pulled off the panel near the wire running to the rear windshield and discovered the "power radio amplifier" had melted and caught fire, resulting in burns to the panel and complete loss of radio and rear defroster. This of course made it impossible to defrost the rear windshield. A replacement part was bought and installed by myself (the wiring harnesses and cables needed to be repaired due to severe burning as well). I still own the old burnt part.