Welcome to po.com
This topic has been beating to death, so I will advice you to get used to the search function.
Anyway, the problem you are having is with the brightness controller. Are you using the stock one or did you get the one that came with the El gauge?
If you did not get the one that came with the El gauge, then you will have to rewire the stock brightness controller to make it work.
pm sil96prelude for more details
