I would like to add my $.02 in here so that hopefully it helps or saves someone some money or possibly time.
I have a 2015 RAM 2500 SLT Cummins. I have upgraded the truck to a full Laramie, yes all down to the wiring harnesses. (I was able to get my hands on salvage truck parts for cheap).
When I converted my single zone to the dual zone, I originally ordered a complete distribution housing. It was on a very long back ordered and I was tasked to deploy so I had to cancel the order. When I was able to start this project back up, I did a bunch of research and ended up finding this product -
Here is an aluminum Blend Door for the 2009-2019 Dodge Ram trucks. This door and driver will replace your factory plastic door and driver. Our doors are CNC machined out of solid aluminum and contain no welds. All of our replacement doors carry a lifetime guarantee. This kit fill fit 2009...
www.blenddoorusa.com
Make sure you click the drop down arrow and select "dual zone".
Once the blend door came in I took my dash and distribution housing out. Took it to the kitchen table (don't tell the wife, but I did put a blanket down so it wouldn't scratch it lol). Completely dis-assembled it via screws and clips, it was actually pretty easy. I took the "single zone blend door" out and replaced it with the blenddoorusa.com "dual zone blend door". Tested the functionality and everything worked.
I had access to the dual zone wire harness and extra blend door actuator. This is the harness that wraps around the distribution housing. I put everything back together and programmed via AlfaOBD, and all worked as it should.
NOTE: I figured since I was already going to have the distribution housing open, I went ahead and ordered the complete blenddoorusa.com kit for our trucks.
4th gen kits are now available! At this time we do not have any installation instructions for the 4th gen kits. In the mean time, you will have to rely on other 3rd party instructions to install these doors. This is a complete kit to repair your 09-19 Ram truck. This kit includes new solid...
www.blenddoorusa.com
At the time I had roughly 55K miles on the truck and the foam on the blend doors was already coming off.
I am sure I am missing a step or two here so feel free to ask away.
So long story short here, I was able to use my original distribution housing and I only changed out the blend doors, wire harness, and added the extra blend door actuator.