Hello I will help you with your question,
They did a good job of hiding the outlet to this drain. Their is a TSB that directs you to drill a 1/4" hole in the round, flat portion of the drain below the HVAC box in the cabin of the truck and clean the drain through this hole then seal it with RTV. I have done this several times and it works well just be careful how deep you drill and use a soft wire or nylon wire tie to clean out the drain.
After cleaning the drain you can install a revised screen on the fresh air intake under the cowling (Part number 55036542) if you wish but this would depend on how often your drain plugs.
Let me know what i can do to help you with this
I have always been able to unclog it through a hole in the drain tube. A nylon wire tie allows you to run up and back into the evaporator case as well as out the drain so maybe they didn't try that. I would not suggest hacking holes in the frame to unclog the AC system, you might need that strength as the rest of the frame rusts.
Nylon wire ties and RTV sealant are available at any autoparts store. When you reseal the hole be sure it is clean and dry and only use enough sealant to close the hole, wait for a couple of hours to allow the sealant to set before getting it wet and you do not want to obstruct the drain with the sealant. There is no pressure on the plug so it only has to close the hole.
Let me know if this works for you