Alright, so i'm trying to relocate an expansion Pak so my N64 will fit in the case and it isn't working out. I have tried a few times with no success so far, but I need to get this to work. I'm guessing the problem is that the wires are shorting? Here are some pictures:
Can't see any problems from the pics, it appears to be a good soldering job. Maybe get out a magnifying glass and examine every connection? You can try running a toothpick between each solder point, sometimes this will remove small solder bridges without damaging the board.
I built a portable N64 and avoided this altogether by relocating the cart slot to the component side of the board. I used 1/8in x 2in aluminum angle for a combination cart slot bracket/ heatsink. Then I had enough thickness to simply plug in the expansion pak after trimming a little off the top of the board.
Sorry if this isn't helpful at this stage, but would be worth a try if you're willing to start over.