99 Ram Heat Help

Ok, I have been suffering all winter and I have had enough, my heat doesn't come out strong. Let me tell you the conditions I have and we will go from there. Heat is available, just doesn't come out strong. If you have the settings on defrost or vent with the fan on low you feel heat radiating out but as you turn the fan up it almost diffuses the amount of heat that comes out of the vents. When the fan is turned up all the way the air temp almost feels cold. Now if you remove the blend actuator and flip the door closed it feels like an inferno the amount of heat being produced and as you adjust the heat/cold knob on the thermostat you can see the blend actuator moving from side to side, but the heat does not come out strong. One more final observation, last night I was riding home from a 30 minute cold drive and noticed something interesting my temp gauge in my car was in normal temperature range. When I would turn my fan up to full power it would actually drop the running thermostat temperature gauge on my dashboard, almost as if a second radiator has kicked in. This is what I thought was odd. One more thing, as the temperature rises outside, lets say to 40 degrees, it does start warming up the truck to the point where I have to shut of the fan down to the lowest setting and even adjust the air mix coming into the cab.
From these observations I got a couple questions.
1. Is there a door to allow external ambient air to go through the vents.
2. If there is what controls it and where is it located.
3. Any other ideas on this would be appreciated.

I would check your thermostat, sounds like it may be sticking. Also check the temperature sending unit.