One problem related to defogger has been reported for the 1998 Mercedes Benz ML320. The most recently reported issues are listed below.
While driving the windshield will suddenly and totally fog in certain temperature and humidity conditions. This has happend several times while driving a warmed up vehicle at various traffic speeds. The defroster is unable to clear this even when it is anticipated and has forced us to roll side windows down and pull to the roadside at highway speeds. This appears to be a design flaw in the defroster because it will take at times 10 minutes to clear and it won't stop the process once it is started even when run at full force.