I have a few gremlins in my heater system. I just bought this Mark III conversion and have never owned one before. It has 5.9 lit.
I had no warm air at all and found the blend door actuator arm had come out of the clip, that fixed I'm trying to troubleshoot the vent selector bug.
Once I select defrost, it is stuck there until; I turn the blower off, select the desired vent floor dash etc.., then I have to cycle the temp from hot to cold, the selected vent will come on then I cycle it back to hot.
Does this indicate a bad dash board controller or just screwed up vac lines?
My next question is, after reading many posts about heater issues I came across a few post about a second heater core for the rear heat. If this is true where is the second core located?
Thank you for any help.

 

Defrost is the default position when there is no vacuum. It sounds like the vacuum is somehow lost or cut off when you select the defrost position. You'll need to confirm if vacuum is present at the vacuum actuators as you cycle through the selector switch.

The second heater core would be in the back of the van, behind the driver's side rear wheel hump.

 

Check to see if one of the hard plastic lines in the engine compartment has been rubbing on a hard surface to the point of wearing through. This happened to me a few seasons ago. It was a black plastic vacuum line if memory serves, and the side of it was rubbing against a heater hose clamp screw to the point of creating a vacuum loss.

If you find a bad spot in one of your lines just snip out the bad section with a pair of side cutters and then insert a small piece of rubber hose between them to restore your vacuum.