2000 Dodge Ram Heater

My heater is going in and out. It sounds like the door is not closing all the way to let the heat in. I got under the dash and sprayed some lubricant on the lever that pulls in and out when you switch from cold to hot. It seemed to be going all the way in and out properly but it will only blow hot air if you give it some gas. When you let off the gas it will blow warm for a minute and then blow cold. If it is the door not opening or closing, how do I get to it and where is it located