Is the rear defrost covered under Hyundai's 5-year warranty?

of course it is and don't let the dealer say its not

Possibly it is covered unless signs of neglect are seen like are the grids scrapped off and if that is the case a dealership could choose to fight warrantying it if they say no to the warranty the grids are serviceable a product to repair the line or grids is available and a person could repair the grids

rear de fogger is covered under warranty. u can very well get it replaced. for elantra almost all electrical items expect bulbs are covered u/warranty. but u have to replace the rear de fogger along with the rear windsheild glass!! it is not a serviceable part.