To me it sounds like the mode door actuator is not working but to be sure remove the actuator and then turn the system on and move the pivot back and forth to see if it makes a change and the defroster starts working.
Here isa guide to help see what I am talking about.
Here is the location for your truck (below)
Please let us know what you find. We are interested to see what it is.
Image (Click to make bigger)
Sunday, November 19th, 2017 AT 1:42 PM