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. :-)

There is actually a little more to it.

Link with pictures

Ain't that the truth about everything... :-)

On both of my mirrors the plastic sleeve was still intact. I did take the first one apart as shown in the linked article, but after doing that I realized it wasn't necessary in my case. If your plastic sleeve is OK, then all you have to do is tighten the 5/16" hex screws.

Thanks for sharing the link. There sure are a lot of message boards for old trucks. It's not like FABO where there's one site that everyone uses.

Happened to me after I replaced them with "PepBoys " brand replacements on my 92 D150. Drove me nuts when going down the highway and they would fold in from the wind. I tightened the heck out of them and it still happened. I finally bought a new pair from LMC Truck and it hasn't happened since I put them on.

I used a thin strip of rubber from a bike tube ,..wraped it around the base of the mirror shaft half way,......that did the trick for me,....LOL

have the same problem with my 2 Dakotas.... the 96 has power mirrors and isn't as floppy and can be electrically compensated for to a point, while the 92 and its manual mirrors drive me nuts.