Sport mirror tightening

My duster has sport mirrors on it, but the passenger side mirror shakes. The base is bolted down, but the mirror itself rattles. The guy that sold me the car said it just needed to be adjusted, but you need to remove the door panels to do it.

So how do i remove my door panels for a 73 duster and once they're off, how do i tighten down that mirror? Is it just a nut below the mirror and i need a real long extension?