Hi, Welcome to Just Answer, I am Clayton. I can help you with this.
I will be glad to help you on this, but we have some conflicting information. Your 2004 truck would not have a DD15 in it. It would have to be a 2008 and up, and the heater configuration will be different from a 2004 to a 2008.
Reply back with your VIN please and we can get this resolved.
Alright, I have the information I need, now for the information you need.
Your HVAC system used small motors to turn the doors to direct the air flow over the heater core.
The most common problem I have seen is there gears lever for the door comes apart or breaks. Pull the lower panel down on the passenger side. 4 screws, torx t25. Take whatever screws it takes to remove the panel below the radio. You should be able to see the actuator motor, it is black, has a 4 plug connector to it, and it is held on with 4 brass screws. Put your finger on this, and move the temp knob from hot to cold, wait a minute, then back to hot. and feel for movement, it should somewhat vibrate a little bit. you can pull the motor off, and watch the motor turn when you change temp settings. Where the motor slides in, you will see a white hex, you can turn this with a screw driver and make the door move. If it does not move, then gears are stripped or broke. There is a gear kit you can buy to repair this.
Check this out and get back with me
where is the heater core located at. The heater cor a nd ac core are the seprate?
Do you have a dieagram?
Yes, heater core and A/C core are seperate. The heater core is more behind the dash.
Here are the instructions to replace the heater core. same procedures to do the door motor,minus the heater core. This will assist you in getting to the motors as well. If the heater lines are hot going to and from the heater core(cab) then it probably is not plugged up.
CLICK HERE FOR INSTRUCTIONS