Brian as you've discovered the blend door located in the heater core plenum or box can also be a problem. You've also found the "easy" way to access the heater core as the factory manual procedure is to first remove the entire dash board!

Here's something I posted a while back: https://www.ford-trucks.com/forums/9...r-problem.html By now its a bit after the fact but maybe helpful anyway.

Your heater core needs to be disconnected from the supply/return hoses which requires the so-called quick connect fittings. These are under the hood, should be easily identifiable. Once those have been successfully removed the heater core should pull out leaving the blend door accessible.

The Quick Disconnect fittings can be removed by way of a specialized tool (OEM/Heater hose disconnect tool (27106) | Heater Hose Disconnect Tool | AutoZone.com) for the job of a suitable pair of stout needle nose or Channel-Lok pliers will work too. At the moment I don't have any images of those fittings as installed, I've eliminated my own since adding a non-factory rear heating system. Here's another link to a thread that might be helpful: https://www.ford-trucks.com/forums/5...nect-tool.html

BTW the blend door and actuator or drive motor are available as separate items on eBay. Ford sells only the entire heater core case, not just the blend door. This of course assumes Ford still isn't selling just the blend doors in 2013?