> the air only will blow through the defrost vent and not from the dash no matter what the dash is set to
This sounds like a vacuum leak issue.
The entire HVAC system inside the cab runs on vacuum lines. It requires vacuum to function. The HVAC box is basically spring loaded to default to defrost mode. The air setting dial on your dashboard redirects vacuum to a series of diaphragms around the HVAC box to change where airflow goes.
The airflow direction mode is not controlled by electronics. It is not controlled by the blend door. It is entirely controlled by a series of vacuum lines, controlling a series of doors not unlike the blend door - but not related to the blend door itself.
I found this picture of the ram firewall online on google images cuz I didn't feel like going outside and taking my own picture. I outlined the master vacuum line that gives the entire HVAC system vacuum:
It enters the cab somewhere above the A/C accumulator. Follow the A/C lines going up into the firewall from the engine side and you should see the vacuum line. It is very thin, made of black plastic, and kind of looks like a wire. Except it goes into a vacuum fitting.
Make sure that you have vacuum at this line with the engine going - engine has to be running, no engine running, no vacuum. If there is no vacuum at this line, then your HVAC box will be stuck in defrost airflow. If you have vacuum here, then the problem is not in the engine bay.
If you have vacuum at that line inside the engine bay, then check the HVAC switch at the dash. That black vacuum line you checked inside the engine bay leads to the back side of this thing:
Pop your dash bezel off and take a good look at the backside of this thing. It is not an electronic switch - it is a simple vacuum port switch. The vacuum lines are all color coded black/blue/yellow/green/etc, so find the black vacuum line that you inspected inside the engine bay and verify that you have vacuum line at this end of that black line. If no vacuum, then you got a cracked vacuum line. If you have vacuum at the black vacuum line here, then the black vacuum line isn't the problem.
Check the general condition of the vacuum block at the back of this switch.
If everything is good so far, then that sucks. If you have vacuum to the back side of the HVAC air switch and it looks healthy at this point, then the problem is inside your HVAC box - and that means you're going to need to yank the dash to take a look at the diaphragms and HVAC box. All of the non-blend-door airflow direction doors in this HVAC box are made of plastic, and they can break and get stuck. The diaphragms that drive them can fail. The vacuum lines that power them can crack... Consider the age of the truck and any of these can go out on you.
Although really with the age of the truck, you probably should set aside a weekend and pull the dash out, replace it with a new one, and clean out your HVAC box anyway... I just did this a few weeks ago, and it was disgusting inside. These trucks have no cabin air filters, so they get full of leaves, dirt, dust, and all kinds of other crap. It's probably due for a cleaning, new dash, new heater core, new A/C core, new HVAC box diaphragms and vacuum lines...
> The air coming from the defroster is cold and never heats up
Either your heater core is blocked and isn't flowing coolant anymore, or your blend door is also messed up. The blend door itself is not driven by vacuum lines - this one is driven by an electronic servo motor. The servo coupler is a cheap plastic ************* that gets broken over time. This is the weakest link in the blend door system. The blend door itself is made of metal though, so at least it won't break. But the servo motor itself could have died, and you can't really get to this without removing the dash and the entire HVAC box - which means venting and opening your A/C system and coolant lines to the heater core...
Spend some time troubleshooting, but all signs seems to be pointing towards dash removal and some general HVAC box maintenance. The truck is getting old, probably needs some attention.
