98 Dodge 2500, 12 valve, bought it new. Now has 80,000 miles. Club cab, loaded, stock. A/C works fine. Cannot get adequate heat in cab in winter, you must have heavy winter clothes on to keep warm. It has had this problem since the vehicle was new. Dealer replaced thermostat and radiator cap, no change.Temp gage comes up barely into normal arc after 30 minutes, but still only cool to lukewarm air out of heater, and no further change if I drive for several hours. Tried covering coolant radiator, no help. Old anti-freeze looked like new when it was changed. Had heater apart and all manual controls and pneumatic valves function normal. Am I the only one having this problem and is there a fix?