FWIW, both of my mirrors (left and right) on my '87 had the same failure: the threads pull out of the mirror post, relaxing the friction spring, resulting in the mirror failing to stay in place. At first it was subtle, with the mirror just feeling like some klutz bonked it in the parking lot, but eventually both drooped on their own and didn't even need highway speed winds.

You can fix it by oversizing the bolt or fixing the threads, or replacing the mirror. About $65 (give or take) from Mama Toyota for a new mirror, or you can go aftermarket for $20 but I have found those to be pretty poor optical quality and the fit on the door is different.

If you embark on replacing your mirrors, know that the cover that goes over the two mounting screws isn't made anymore, and isn't easy to find. Translation: be careful and don't break it when trying to get it off. Make sure it's warm so it isn't brittle in the cold, and then I found I could pull on the upper edges to get one of the upper latches free, and then push in on the bottom edge to work it free. Someone else may have a more prescriptive formula for removal.

Toyota left side: 87940-89135
Toyota right side: 87910-89135

Pics of the right side below:

 

Toyota should have molded into the plastic a little arrow showing you where to start prying. That mirror cover has been a pita since inception.

 

In the hopes these pictures help someone else, below you can see 1) the old friction system, 2) the new friction system, and 3) where the engagement points are on the LH mirror cover.


Original friction system

New friction system

View of LH mirror cover engagements