Heat Control Valve Location
 

94 Yukon 6.5 diesel. Where is the heat control valve for the cab heat located? i have very little heat in my cab when it is sitting idling . No heat and cold air when driving down the highway. When sitting idling the heater core gets hot. Thinking that the valve is not shutting the cold air off all the way and its by passing the heater core somehow.

Any ideas on this guys?