The car is at operating temp. and I have the window defroster on. The passenger side vent blows air that is much warmer than the vent in front of the driver. Furthermore, if I pull up to a stop light the driver side vent starts blowing cold air while the passenger side vent blows luke warm air. Can anyone explain to me why the two vents would be at different temps. and why there is a sharp drop in temp. whenever I come to a stop? (I am new to diesel engines, and the only frame of reference I have is the chevy blazer I used to drive)

 

Its a leaking MONO valve diaphram if you have auto temp horizontal buttons on the dash. The rubber diaphram tears and wont hold hot coolant in the heater core. Buy a MONO valve repair kit. The valve is above and to the right of the battery.