2003 Dodge Ram 1500 quad cab. While replacing a leaking heater core (since the dash was removed anyway), a local mechanic attempted to correct a 2-year-old problem with my defrost: the selector switch does not direct air to the defrost vents. The system blows strong and hot/cold to dash and floor, but just does not blow through the defrost vents. He replaced/repaired the blend doors/housing/selector motor assemblies (blend door hinge pins and/or pin housing were broken/cracked). It seemed to work at least once after the repair. But after running through the selections on the HVAC switch (floor, dash, defrost) a couple of times, it again failed to blow to the defrost. During the second attempt to repair it, he first verified that the fixed/replaced blend door unit repairs/replacements had not broken again (they were still OK). He did realign/reattach the blend motor drive motors and reassembled the dash. I picked it up the 2nd time and had the same results...ran through the various settings a few times, got the defrost two blow once/twice, but soon the defrost failed to blow to the dash again. Any suggested alternatives to pursue (e.g., maybe a weak vacume problem)?
Possibilities, but would either of those potential culprits explain the intermittent nature of the problem (i.e., it worked for him, it worked for me when I picked up, but it stops working soon after I leave and start running through the dial settings)? Note that it's been doing this since long before the heater core replacement (on occasion, if I left on defrost long enough, it would come on, but it would "stick" in that position, then it would mysteriously switch back to floor or dash only again later and not go back to defrost). So, I suspect nothing they did caused or exacerbated the problem...their recent attempts at the diverter box/valve/drive motors just didn't fix it yet. He is being helpful and he sounds more frustrated than me since my next trip will be the 3rd time in a week he's removed my dash (uggg...my heart goes out to this poor guy)! So, I wanted to offer any suggestions I could to help him nail the issue before another tear-down. Thanks for any additional thoughts...I'm dropping it off again Friday AM...fingers crossed.Possibilities, but would either of those potential culprits explain the intermittent nature of the problem (i.e., it worked for him, it worked for me when I picked up, but it stops working soon after I leave and start running through the dial settings)? Note that it's been doing this since long before the heater core replacement (on occasion, if I left on defrost long enough, it would come on, but it would "stick" in that position, then it would mysteriously switch back to floor or dash only again later and not go back to defrost). So, I suspect nothing they did caused or exacerbated the problem...their recent attempts at the diverter box/valve/drive motors just didn't fix it yet. He is being helpful and he sounds more frustrated than me since my next trip will be the 3rd time in a week he's removed my dash (uggg...my heart goes out to this poor guy)! So, I wanted to offer any suggestions I could to help him nail the issue before another tear-down. Thanks for any additional thoughts...I'm dropping it off again Friday AM...fingers crossed.
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Leaking heater core or lines most likely the core cracked or was punctured. Replace your heater core since during removal you can inspect the lines for leaks
If it is necessary to remove the heater assembly, the cooling system must be drained before removing the heater core.
When a heater core leaks, a new heater core is installed or the old one repaired. Heater Housing
The heater housing is usually under the dash and must be removed to gain access to the heater core.
Procedures for replacing the heater core vary with the year, make, and model of car. It threfore necessary to consult the manufacturer's repair manuals for the proper procedure for replacement.
The heater housing is disassembled to get to the heater core.
Heater Core
Remove the access panels(s) or the split heater/air conditioning case to gain acess to the heater core.
Remove the heater coolant hoses.
Remove the cable and/or vacuum control lines (if equipped).
Remove the heater core securing brackets and/or clamps.
Lift the core from the case. Do not use force. Take care not to damage the fins of the heater core when removing.
To reinstall the heater core, reverse the removal steps.
When the heater core leaks and must be repaired or replaced, it is a very difficult and time-consuming job primarily because of the core's location deep within the firewall of the car. For this reason always leak test a replacement heater core before installation.
You have to pull the dash almost out to replace the heater core. first drain the coolant and remove the heater hoses in the engine compartment. Next lower the steering column. there are 4-5 8mm screws onthe top of the dash,and tw 13mmbolts on the sides remove the kick panels to see them. Also remove the airbag module on the tranny hump under the dash.when all is unfastened you can pull the dash back on the passengers side back and almost put it on the seat. you will see the heater core on the top of the heater box. Discharge the ac and remove the heater box fasteners in the engine compartment . pull the box down inthe cab of the truck r&r core. sound worse thatn it really is good luck
The heater housing is disassembled to get to the heater core.
Heater Core 
Possibilities, but would either of those potential culprits explain the intermittent nature of the problem (i.e., it worked for him, it worked for me when I picked up, but it stops working soon after I leave and start running through the dial settings)? Note that it's been doing this since long before the heater core replacement (on occasion, if I left on defrost long enough, it would come on, but it would "stick" in that position, then it would mysteriously switch back to floor or dash only again later and not go back to defrost). So, I suspect nothing they did caused or exacerbated the problem...their recent attempts at the diverter box/valve/drive motors just didn't fix it yet. He is being helpful and he sounds more frustrated than me since my next trip will be the 3rd time in a week he's removed my dash (uggg...my heart goes out to this poor guy)! So, I wanted to offer any suggestions I could to help him nail the issue before another tear-down. Thanks for any additional thoughts...I'm dropping it off again Friday AM...fingers crossed.
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