So I finally got around to adding the illuminated buttons, and now the display no longer works properly. The symptoms are:
when in AC mode, the display is very dim. in car mode, the display is very distorted: i can somewhat make out what is on screen, but it's very difficult.

on the bright side, the buttons do light up. if anyone experienced something similar during their installs, please let me know. i hope i didnt fry anything expensive.

regards
-ted