I decided to put together an all white themed Game Boy with frosted clear white case using parts I'd scavenged from old manky DMGs. In this particular one, there was a bunch of vertical lines that I fixed before going in for the backlight. The screen was stuck down pretty well, and I must have done something to the cable while I struggled to get it off as there was a horizontal line dead once I had the backlight installed. Eugh.

I thought it might be pressure on the screws or screen, but nothing helped. Since I was going to have to confine the damn thing to the scrap heap anyway, I decided it wouldn't hurt to give the white ribbon cable the old soldering iron rub treatment and see if it did anything. I knew it apparently melted really easily, but I have a temperature controlled soldering iron that I invested in a while ago when my first shitty one died. I tried it on the lowest setting (around 160C), and rubbed it over the length of the white cable, but no joy. The line was still there. I decided to bung the temperature up a bit (to about 210-220c), and then rubbed it all over the cable - all along the length, up to where it connected to the screen, and back down - everywhere basically - holding for about 1 or 2 seconds tops. After a couple of goes at this, I slid the polarising filter back under and the damn line was gone!

I'm not sure if this would work for those screens with bigger horizontal problems, but since most of the time DMGs with these problems get chucked away, it's worth a bash!

TL;DR - to fix horizontal lines, before throwing the DMG away as a write off, try setting an adjustable soldering iron at 200ish Celcius and rub all over the horizontal white ribbon cable.