So per your request, I tried to take a photo of my mirror -- but the place where I put the wedge is hard to get to. Here is the photo, and then I will try to describe what I did:
Click For Full-Size Image
If I posted that photo correctly, there should be a "click for full size" button somewhere. It may be easier to see if you look at the full size picture. And if I recall correctly, when you click on that button, it takes you to a fairly small photo, and then you have to click on that small photo to really expand it.
Anyway, I am not sure if this description applies to all Toyota products. But on my Tacoma, the mirrors are mounted on base units that attach to the body of the car. The mirror portion is hinged. I'm not sure why the mirrors are designed to pivot on those hinges -- maybe for really narrow parking spaces??
The problem is that the resistance on that hinge is fairly low. Give your mirror a tug and you will see what I mean -- it is easy to make the mirror pivot toward the body of the car.
Now, if you add strap-on towing mirrors onto the existing mirror, you've got a lot of wind resistance pushing on the hinge. Add in a big headwind and the breeze of big-rigs coming at you on a two lane highway, and the mirror suddenly folds inward. You then have to open your window and push the mirror back into position, where it will stay until the next truck passes.
Important tip -- if you are wearing a baseball cap, take it off and tuck it under your leg before you open the window. Don't ask how I know this.
So to combat this design defect, I made a very narrow and sharp wooden wedge. I jam it into the hinge assembly, right where my finger is pointed. As the mirror starts to swing inward, the friction on the wedge prevents the mirror from moving. I tap the wedge into place with a small piece of wood. Problem solved.
Except that sometimes the wedge falls out due to vibration.
So I bring extra wedges.
It is not a perfect solution. I did try hot glue -- it did not hold. I suppose I could try Gorilla Glue, although I have found that in high vibration situations it gets brittle and chips off.
If anyone has any ideas for preventing this problem, both wverain99 and I would appreciate it!!