If you did all those things I mentioned and clicked the install button describe exactly what happens next. The installer is just a standard .pkg installer (inside their installer app bundle) which should uncondititionally write (or rewrite) whatever and wherever the .pkg is instructed to install. No previous uninstall necessary.
Look at your console log as you are installing for any related messages be posted.
Adobe recommends otherwise - Uninstall Flash Player | Mac OS
They are just being conservative. I've looked at their .pkg and if it wasn't for the fact that they used .lzma compression* on the plugin I would have just recommened using pacifist to extract it.
* Tried some lzma utilities but nothing decompresses it. The flash installer has a postflight that has their own lzma decompression.
Hi and again thanks for your help! yes, I did that many times and bascially I keep getting a mesage saying that the installation was successful...
You don't even have to try running youtube. Just look at /Library/Internet Plug-Ins for Flash Player.plugin. Indeed, keep the folder open while running the installer and see if it appears at all and then disappears shortly thereafter.
Also, while it makes no sense why the installer would be running to completion successfully without installing, for the sake of experimentation I suggest trying to install from another account.
O have ALWAYS this problem with Adobe Big Brother
I just solved my latest issue (updating manually from 11 to 12) by downloading & installing from here:
Have fun !