I'm guessing I'm not the only one who picked up an old Boxster with visors that won't stay up. The problem with mine is the ribs on the post and in the post hole wore down, so there was no longer any grip keeping the post from turning freely in the hole. This is my fix...
1. I removed the bracket and visor.
2. Drilled hole in the back side of the metal bracket, and into the post hole. I drilled a bit into the other side of the hole wall, too, being careful not to drill all the way through to the front.
3. Inserted the visor post back into the bracket, making sure my visor was turned properly, then re-drilled to put a hole through the visor post.
4. I cut a pin from a coat hanger and pushed it through the hole, locking the visor and bracket together.
Here are some pictures...
Drilled hole
Drilled through into other side, to make the pin hold even better.
Hole through visor post
Reassembled visor and bracket, with pin in place