Normaly a led rope is a 12Vdc so you can solder it directly to the background illumination plug. On the schematic there is 20Vdc at the rope plug and led works with a voltage between 12Vdc to 24Vdc.
As you can see on pictures the rope stop lightning when eyes of Ra are on.
I bought the pinball with this update and I think it work well but cannot help you more by looking on the display cause I'm about to restore it and unplugged the playfield.
Look at your led rope tranformator and if the output is a 12vdc you can cut it and directly solder the led rope without the tranformator to the backbox plug.
Hope that help you.