If you have an older MX-500, the backlight button often broke over time. You can open it up and fix it, but you'd be better off to give URC a calll at 800-901-0800 and send it in to them for proper repair. I think they charge about $15-20 for the backlight repair.
If you must do it yourself, the way to open the MX-500 is to grasp the top and bottom of the remote with each hand, then twist each end in opposite directions to pop the snaps between the two halves of the case. This is not easy and you'll have to twist it so hard you'll think you're going to break it (and you may if you're strong enough), so it's not recommended.
Surf Remote Control