Unnecessary IMO. I never installed mine. It was plenty bright back there. Particularly with the additional lamp installed in the right most lane guide.

This feature was in the early games, but dropped later on.

You need the "lost plastic", which covers most of the backboard, and you have to cut a slot for the light to go through the backboard.

The light will make the bridge plastic noticeably brighter.

I think adding an LED strip up under the backbox ridge is much more effective. I also added a light to illuminate under the bridge. It sits under the left ramp screw and standoff. I find this method much more effective.

I just installed the lost plastic with the pinbits LED lighting. The LED lighting itself is pretty easy to install though you have to hunt around the pinbits website for instructions (they are linked from an earlier version of the kit). The hardest thing is cutting the hole in the backboard...if you don't have a jigsaw. I had to drill a series of holes and then sand them for ages to make it smooth. They provide a template to make sure you do in the right place.
I haven't quite finished putting everything back yet so I can't quite judge how good the lighting is yet; but early indication at least is that it's nice and bright (thanks to the LEDs).
My main problem has been installing the lost plastic - it's pushed the rope bridge rail forward a few mm and now the pop up post that stops balls for the POA is stuck as it's rubbing against the side of the hole. I think the solution might be to adjust the below playfield solonoid which I will try tonight.
Anyone else have this problem and have a solution??