I didn't think this system was vaccuum drivin. Not 100% sure.
But after doing a couple hours investigation.
I found a website http://www.heatertreater.net/Ram%2002-08%20Listing.html
They found common issues with the blend doors in these Ram's. Below is from their website.
3. Mode Door 1.
a. This door regulates air flow to either the dash vents or defrost/floor. If you have air flowing only through the vents with no defrost or floor, this door is failing. From our experience, this door is usually the first domino to fail. Not sure why, just is.
Looks like a better solution for the typical garage mechanic who does not want to pull the whole IP out of the truck.
Anyone please respond