Did you find a solution?
The other Lexus forum discussing this is “Side mirror problem on 2013 rx 350“ they identified the clip that needed replacing. We’re you able to get it?
Thanks

 

I ended up having the entire mirror replaced. Cost a few bucks, but went with original OEM and it's been working fine since.

 

I had the same issue, took it all apart, though was not able to fix it myself. So I found Theo, who fixes all mirror motors, wiring, gears by tearing it all the way down and replacing whatever is broken, fixing all makes and models. While I was at his shop I did see the ring pictured above, so I know he has the part. Here is his info (Tell him Greg sent you):
Texas Highend Auto Center Inc
Theo Kha
972-429-0857 - Work
214-264-1577 - Cell
23 Steel Rd
Wylie TX 75098