Here's a quick little guide on how to add a couple of led's to a GBC cartridge.
First, you're going to want to add just a little bit of solder to one of the blue contacts and one of the red contacts in the second picture below. Red is ground and Blue is VDD.
The current going through the cartridge is 5 Volts, so make sure you use correct resistors for your led's.
Since I used 2 3.3v blue led's, I needed a 47ohm resistor, and it worked out perfectly. I would recomend using the resistor on the VDD b/c there is a little more room. (if you don't know how to calculate what resistor you need, use this: http://diy.sickmods.net/Tutorials/Case_Modding/Resistor_Calculator/ Though this site is very useful, I would just use this to figure out what resistor you want then head over to ebay to buy, everything is cheaper on ebay )
I used a small piece of PCB to wire everything b/c I think soldering to them is easier than soldering wire to wire, but it is NOT needed, so don't worry if you don't have any.
Once you've wired everything together, I wouldn't be afraid to use electrical tape on any of your handywork, better to be safe than have a short.
If you have any questions, just comment and I can help.
