yeah that box with the cable sscrewed into it. i think it has 2 wires going to it and bolts to the inside of the tailgate. you can try opening the window with the tailgate barely opening to make sure its not wedging into the weather stripping, just dont close the tailgate unles the window is all the way down. my last k5 was so jacked up that winudow wouldnt go up unless one hinge was fully locked and the other one only locked one click other wise it would wedge on the roof section and stop halfway up

 

I have a 1986 as well and am having the exact same problem. Was your final conclusion?

 

I had the same problem but mine wouldn't go up. So I took the transmission apart and noticed a washer had come apart inside and james the gears prohibiting from moving at all I removed the broken washer greased it and I was in buiness.