Where is the heater receptacle on a 1999 F-350 diesel?

There isn't a single "heater receptacle" in the way you might be thinking on a 1999 F-350 diesel. The heating system is integrated and doesn't use a separate receptacle like a portable heater.

The heat comes from the engine's coolant system. The coolant is circulated through a heater core located inside the dashboard. A blower motor then pushes air across the heater core to warm the air inside the cab. There's no external plug or receptacle for a separate heating element.