I know this is an old post but I wanted to pass this on because it may be relevant to someone else now. You described exactly what my truckwith 5.7 Hemiwas doing.At first, it would occasionally act like a stuck thermostat so I changed the t-stat and did not help. Finally, coolant loss got bad and the smell of anti-freeze was in the exhaust.....Even with full coolant, the loss of hot air from the heater washappening. Thiswas caused by loss of compression through the headgasket...gases/air being injected into the cooling systemwhich in turn was causing the water pump to cavitate, and no flow of water through the engine and heater core. The engine temp would spike and heater would get cold, then the pump would catch prime again and engine temp/heater would return to normal. New heads fixed the problem under warranty.

 

the key on off on off will work on every 2nd gen ram EXCPT most 1998's and earley 1999's