I was going to say Heater Core as well...my fathers 95 Dodge RAM had this same issue and you always knew it had to be the heater core.
2007 Smokestone LX HPP
05 PI Airbox, BOC 91 Octane Tune, Power Slot F&R Rotors, Police Front Springs, Raybestos Police Patrol Pads F&R, 260A Alternator, Ported Intake Plenum, Adco Front & Rear Swaybars, Metco Control Arms, Metco Watts Link, PI Shocks, Walker Quietflow 3's, 3.55 Locker, 180 T-Stat, 22C Plugs, Pirelli P6 Tires
1999 PI, Former San Antonio PD Unit #171
1999 LX (wrecked)