I had to solder a wire from GND 32 to 103 on the back of the board and the picture is 100% perfect.

Did you test the pins of the pot to make sure your wired it to the correct pins? Did you wire the negative or the positive to the pot?

The trimmer (pot) is on the GBA's main board, hopefully nanode didn't solder anything to it - you don't need to for this mod.
The trimmer is just used to control the signal level to the LCD screen I believe.

The pic you posted up looks like it does only just need some adjustment with that trimmer for sure. It's the same dim colours I got when I installed my AGS-101 screen.

With the 40-pin adapter cable you should solder a wire from one of the pads on the adapter cable to a point on the GBA's main board.

Grab the instructions PDF from this Gist and double check.
The Gist says you may need to solder a cable to ground to avoid washed out colours.

Good luck!

So I'm trying to transplant a working mainboard into a case with an AGS 101 backlight. It's 40 pin and the new board that I have IS compatible and is just a newer/cleaner version of the previous.

I'm at the point where I have to test the voltage and brightness of the backlight and I'm running into some trouble. No matter how far or what direction I turn the potentiometer, the backlight stays at the same setting of "way too bright to see anything."

Ref:

Is there a known fix? If it helps; I don't have a soldered ground connection from the ribbon cable, but I don't think 40 pin cables need one if my knowledge serves me right.

Any help would be greatly appreciated!
-Sam