Installed my Grumpy light mod today. Super easy and just as described.
I used a purple light strip on the metal deflector behind the drop targets for the added GI. There is some reflection of the mini-playfield, but I can live with it. If I decide to change that to something else, it should be easy.
Used another purple light strip on the back of the plastic enclosure in front of the kicker for the activated light.
I like it. You really notice when the extra light comes on and the mini-playfield is very lit up. Also, since the GI turns off at certain points, you really notice that.
Grumpy also packed the extra $2 that I accidentally sent him in with the mod. What a nice guy, he doesn't seem Grumpy at all!