How do I Tighten Side View Mirror 1968 Cougar?

How do I get the Mirror tight in the drivers side mirror assembly? The mirror drops down in the mirror assembly when I drive. I can’t see a tighting screw when I move the mirror to one side or another in the housing. This is a standard mirror and is not a remote mirror.
Anybody have a good original 1968 drivers side view mirror for sale?
Thank you,
Ric G. In Cali
805-279-5688

Easy fix, take the mirror loose from the door, look inside the hollow base, locate the bolt and tighten it.

https://youtu.be/B7ae4_n4dEA?t=544

That video tells it all. When you’re in there, look for stuff a previous owner may have done to “improve” that third screw - I found a non-original serrated washer that was actually making it MORE prone to getting loose.