Side View Mirror Disasembly question?

I have been looking around my truck in preparation for some body work and notices some small rust holes in the metal housing of my side view mirrors.

upon further investigation I have found that they are rusted badly.

I have a plan to modify the housing (I am pretty good with sheet metal and I am a tig welder by trade) but when I tried to disassemble them I ran into a problem.

I can't figure out how to remove the moving mirror part from the housing. I have taken the rest of it apart and I cant see anything that will release the mirror from the housing.

anybody ever figure this one out before?