Two problems related to defogger have been reported for the 1999 Chevrolet Tahoe. The most recently reported issues are listed below.
Heater/air cond. Fan switch fails to actuate fan intermitently, causes windows to fog up during rain or cold, in winter can not defrost winshield. Have noticed same problem in other Tahoe's from 1998 and 1999 production years. Creates hazerdous visability during driving.
Three individuals at my work have had the knob fail which prevents the defroster from working creating a hazard with iced windows. Dealer was smug and noted that they replace lots of these. The failure of the knob is internal to the module and prevents an easy inexpensive fix. Repair is 285. 00.