This allows you to switch from ENG <--> JP
-->You will need:
-->Philips head screwdriver
-->SPDT Switch (I think it's called that. It has three connection points on the bottom)
-->Drill with Drill-Bits
-->Thin gauge wire (Three different colors)
Please read ALL of the image notations, as they are of great importance.
Step 1: Time to Take Things Apart!
Step 2: Find Your Working Area.
This part is very tough. I recommend that if you're not very goodworking with small electronics, STOP NOW!
Locate the small area we're going to be working with.
Step 3: Preparing the Work Area
Here's where the really hard part comes.
Locate Pin 107, and lift it with something very small and sturdy.
Locating this shouldn't be too hard; there's a 105, and it's counting from there.
After that, you can put some tape under it.
Step 4: Soldering, Soldering, Soldering!
Add solder to pin 107
Add some solder to the end of 108's route. If your Genesis doesn't have a nice convenient copper plate, you can scratch away to expose some of the copper.
Add some solder to the ground area.
Step 5: Preping the Wires.
Strip one end of the wires.
Add solder to the stripped ends.
Step 6: Connecting the Wires
Just solder the wires to the pre-determined spots.
Step 7: This Is Optional.
Again, this is completely optional. For aesthetics as well as caution.
Step 8: The Switch
Make them even!
Strip and solder. Refer to Step 5.
With the switch, add solder to the three connection points. Go nuts.
THE WIRE FROM PIN 107 WILL GO IN THE MIDDLE!
Step 9: Placing the Switch
I happened to give it a circuit-bent look.
Step 10: Modifying the Cartridge Bay.
This just makes it so that Japanese games may fit in the cartridge bay. That's all.
Step 11: Re-assemble
Step 12: Testing the Results.
You can just switch back and forth between regions.
Have fun and leave feedback. :)