I am having the same problem and have tracked the heat part of it to a faulty heater control valve for the rear heater core. This is located under the van on the drivers side. The way to test this is as follows: Disconnect the white vacum hose from valve- start car and wait for engine to warm up -feel the hose coming from the engine to the heater conrol valve it should be hot . now feel the other side of valve if it is hot the valve is stuck open and needs to be replaced. If you chose to do this yourself be sure to drain the radiator first. As far as the air blowing out only the bottom I agree with previous posts. Good Luck | 
