Defogger Heater problems of the 2007 GMC Envoy

2007 GMC Envoy. Driving on intrstate on Saturday 1/8/11, cold snowy day, all of a suden steem started to come out of vents, all windows fogged up could not see and strong smell oof antifreez. Had no place to pull over. A good thing my wife was in pssenger seat she took off her coat and keept the winshield clear till I could exit the highway. Took car to dealer and was told the heater core was leaking. Thisa could have caused a bad accident if we did not act quickly.