96 dodge ram heater problems

Have a 96 ram 2500 4x4.Has the V 10 engine in it.According to the temp guage engine is warming up to normal operating temp's usually around that 200 degree mark.Problem is the heat that is coming threw vents is only luke warm.On cold winter days that isn't warm enough to do a good job of defrosting windows,unless your driving major distants.Wondering if heater core is plugged.Or if the valve that controls the flow of coolant from engine to heater core, is not opening all the way.And if so where is it located.Any other ideas on what could cause this problem.Thanks for any and all help that you can offer.