brightness slider not working [not relevant]

It doesn't require 'activation' as such. The age old and obvious question is; have you rebooted/powered down the phone and tried again?

It helps others to help you , if you give more information with your question, like telling us what you have been trying to do to remedy the problem.

There is a 'tool to access via your telephone keypad, called 'CSD Tool', in there you can run a single test to see that the LCD dims - I realise this isn't quite you're asking for, but a process of elimination is often required to get things sorted.

To get CSD Tool, on your telephone keypad, dial *#*#310#*#* or type 'CSD' in terminal.

When CSD Tool opens, select Single Test > LCD Backlight. If you get a pass (the screen dims), we can assume there is not actually a physical problem with the LCD and a reboot then try the slider again - I'm guessing at this point. There may well be some lines to be tried in Terminal, I'll leave that to the more experienced among us.

Regards

And you probably know this, but I'll mention it just in case: There's a delay in the brightness change, so you don't see it immediately when you move the slider. Try moving it down to the lowest setting and in a few seconds the screen should gradually dim. (Actually, I don't know why it's like that, I'd prefer an instant change.)