I thought I'd post this in case it might help someone. On my 1989 D100, the side mirror is on a post that allows it to swing fore and aft for adjusting the view. You are supposed to be able to grab the mirror and put it where you want it and it should stay there. Problem was, at highway speed the air would push it back so I was looking at the rear of the cab instead of traffic in the other lane.

On the bottom of the mirror base there is a small Torx screw. Remove it, then gently pull up the base cover and slide it up the post. There's a rubber O-ring that seals around the post; be careful not to break the thin lip that holds that O-ring in place (ask me how I know it will break). Under the cover you will find two small hex-head screws that adjust how tightly the post is clamped in place. Tighten those until the mirror is pretty hard to swing fore and aft.

I think the same mirrors were used from 1988 to 1993, according to RockAuto. :-)