[Help!] Game Boy screen acting up after mod


I was asked to add a backlight mod on two Game Boy Classics that already had a VGA mod added. Since the VGA mod was put on the back-part of the Game Boys and the backlight mod I was to do is going on the front-part I decided to go for it.

I've tested the first one now and it's acting up on the screen - Vertical rows are glitching out in different opacities - this is NOT pixel-problems since I did check this before modding.
Later I've also checked to see if the screen worked with a normal back-part. It's still acting up with those weird vertical lines of different opacity...

Is there anything I can do? It seems like the VGA mod has it's own power-supply cord. Maybe it will look fine when this is attached?