clip on rod for heater box ?

[Daves72'GMC]

On the metal heater box inside the truck,where the heat comes down to the floor vent right over the hump, there is a rod that goes through the metal box which opens the flap inside the box and on the back side of the metal box where the rod comes out the back is there some kind of clip that keeps the rod from pulling out?.The rod on mine keeps coming out because there is nothing on the end of the rod to keep it from coming out.Where can I get this clip or whatever piece I need to fix the problem? Thank you all.Dave