(Original modification discovered by Trash80)
Flip your gameboy over so that the battery cover door is facing you. Open the battery compartment and set the Bat. Door to the side. Now all 6 tri-wing security bit screws should be exposed. Remove them.
The side with the shield dressed out will be connected to the pot. Note polarity so when/if you pan things left and right they actually come out left and right ;) Be careful not to make and shorts. Be sure to tin your wire and apply a little extra solder to the pot terminals so that they connect very quickly when reheated. It should look something like this....
For the internal modification all you need to do is cut the two far right wires (if the cart slot is closest to you) which are the output from the headphone amplifier. I removed them completely so they weren't dangling around inside.
Now take a look at that 4 wire connector that connects the output board to the main board. From left to right the connections go: Switch normal (the line that cuts the internal speaker when you plug into the jack) Ground (GND), Right output, and Left output. These are you points that you will solder your shielded cable to (Right/Left output). You might be wondering why I chose this spot instead of directly to the jacks. And to answer this question, it's because there are output de-coupling caps on this board which will assure no dc-coupling issues with other gear. It's just a precaution. I have tried soldering to the jack and to these points and found there to be no different in audio quality so it's safe to use these points. Watch for shorting when soldering. Again, pre-tin your wires and solder points for a cleaner job.
Step 5: Dress the wire so that it doesn't get pinched when putting the gameboy back together. Test your output by reconnecting the Display ribbon cable and playing a game or a track. Check polarity as well. Remember, you're checking with a line level input not headphones. If it works, CONGRATS!!!! J your gameboy classic is now a lot louder and will punch a little bit better. If it doesn't work. Double check your work and read through the steps again.
If you choose do do the mod externally there are some slight differences in the wiring. Both sides of the cable you are install must have the shield dressed out (the shield on the output side will get soldered to the casing or ground point on the plug). And the only wire you need to clip on the output board in the GameBoy is the white wire (which is the normal for the internal speaker). This will deactivate the internal speaker so you don't have to have something plugged into the headphone jack if you are using your external output. It will look something like this...
So if all goes as planned you should now have a Pro Mod'd GameBoy. Sweetastic!!! Feel free to email me with questions should have you have them!
And if you get really clever, install a footpedal jack to hit the start button remotely! :) it's great for starting a track with your feet if you are using your hands for something else! I might do a how-to for this but it's really simple. just trace the start button pads back to a spot where you can solder wires to. the wire them like you wired the external outout :)