I did the mod as described. I moved the switch and connected the green wire to the black ones. Now when I turn it on, the screen opens, but stays blank.

I reconnected the green wire to its original place and the screen is still blank.

Any ideas where I screwed up and how I can fix this?

Thanks for the help!

In case this helps (and I am a complete idiot), I grounded the green wire going TO the N3 unit. Is that one not supposed to be connected to anything and should I have grounded the green wire going into the car?

It is fixed. I disconnected the wrong green wire.