Heater blows cold air
 

My heater will only blow cold air. Over the past several months there were a couple of times I thought the heater simply wasn't warming up, but most of the time it was great so I thought it was just me being extra sensitive to the cold. We replaced the thermostat for other reasons, which solved that problem, but now the heater never warms up, it only blows cold air. I've been given a couple of suggestions, 1 - temperature control module (which when I search for price listings says does not apply to my model of vehicle) 2 - heater core, and 3 - the "flap" that switches from a/c to heat could be stuck. Is there a way to determine what is the problem without ripping out the dash of my van? We've taken off the underneath part of the dash but still cannot get to it all, and the whole dash seems like it will have to come out. Is there something that would help indicate to me what the problem really is?
OH! I have a 2001 T&C Ltd

First make sure the heater hoses going to the fire wall are hot hot enough u cant hold them.

I have the same problem with my 2000 5.9L durango. The heater blows a little warm air on a low fan setting but goes cold if I turn up the fan. The heater hose is hot suppling the core and slightly cooler leaving the core but the core is just about 6 years old. This lack of heating started after replacing the water pump. I am wondering if it has something to do with the bypass hose on the water pump. It seems to me the rad fluid would rather bypass than go through the heater core. Can the heater core plug up sudenly after 6 years?

yeah if u had something come loose from there