The heater box assembly is under the passenger seat. Take out the seat, then that plastic cover/tray, then the steel lid and it is all there. The door comes with the housing as an assembly if you ever replace it. Easy job, hardest part is getting the seat out of the way.
The very first thing I want you to check and service are the drainage tits which are located under the cab directly above the fuel tank behind the angled panel. These if not cleaned out become plugged and will affect the proper drainage of condensation from the evaporator. There are two of them within a foot of each other. Reach under there and squeeze the bottom of tits to open them and allow dirt or mud that's accumulated in there to fall out. The second part to have looked at is the expansion valve next to evaporator. Third is check for low freon, and forth is the receiver dryer which sits outside the frame rail behind right steer tire. The reason I haven't suggested the blend door actuator is because I've never had one fail. The way to test actuator is turn ignition key to on position but do not start engine. Next turn the cold to hot from far left (cold) to far right (hot) and listen for blend door actuator to cycle then repeat the other direction. You will hear a slight buzzing sound as actuator moves. If you want visual verification, simply remove bolts from passenger seat, unbolt seat belt bolts to seat then rotate seat to back sleeper area leaving air hose connected. Then unbolt the steel plate cover and move out of way. Once again rotate the blend door actuator and physically watch the blend door move from one extreme to the other. If it moves, blend door actuator is working. If the door doesn't move, simply unplug actuator near front left corner above blower. Remove screw nuts, and gently lift actuator straight up allowing armature below it to fall straight down. Do not force that white linkage armature, it just sits in the gear below the actuator. Get a new actuator, line it up with mounting holes, place white plastic armature linkage back in the location on the gear below the actuator and place in mounting position and replace screw nuts. Plug in actuator harness and test with cold/hot control switch on dash climate control center with key on, engine off. Once verified, replace cover and seat. Another item you might want to do is turn off both your water shut off valves during warm weather and open during cold season. One is located behind water pump, second is located behind turbo charger. They are just like the one's at your homes outside water supply spigot. There are five items I've listed here, one of them are guaranteed to be your problem but be sure to clean out the condensation drainage tits every single year. A/C system maintenance is very easy but you need to be aware of what to check for, it's as simple as that.
