Hello everyone!
In this tutorial, I'll be adding two RCA phono sockets to the back cover of my Game Boy Color. Now, because I've already had an internal ProSound mod wired to the stock mini-jack, I'm gonna do the same for RCAs. I've had my speaker removed, so I had a little bit extra space to fit in the socket. Can't be responsible for folks with speakers.
Both outputs are working at the same time, so while RCAs are plugged into your mixer/any other piece of equipment, you can use mini-jack for monitoring your tunes! 
For pictures please visit my Tumblr
First things first. We're going to need:
- 2x RCA sockets. (Unfortunately they didn't have silver ones so I bought gold ones).
- Drill, dremel tool, round shaped file, sand paper. Basicaly anything that'll help you to make two holes for your RCA sockets.
- Some different colored wires.
- A soldering iron and some solder.
- Tri-wing and + screwdriver of course.
- Hot glue (optional)
First step:
Remove first six screws from your GB and remove the back cover. Now, take your RCA socket and try a few various positions for it on the back cover. Your sockets may be bigger or smaller that mine. Depending on that you should decide upon a place for them and mark two little dots on the back cover - theese are gonna be two centres for future holes.
(Make sure that the lenght of the socket is okay. Mine are just touching the main circuit board)
Step two:
Use your favourite tool to cut out the holes. Make them as neat as possible
I used 6.5mm drill and then widened it a little bit with a round shaped file. Remove all the little cutouts of plastic and blow the plastic powder away. Be careful not to get it in your eyes or nose, wear safety glasses and respirator if needed.
After you're done, you should end up with something like this:
Step three:
Before I put my RCAs in I connected both grounds and added a longer wire, which will be connected to a ground on the PCB.
Fit your RCAs through the holes. Don't forget about L/R coloring if you have two different colors, decide which position is suitable for you. If you cut the perfect hole, they should just slot in nicely and don't move from side to side. If you're like me and you like lot's of hot glue everywhere, apply some hot glue around the connectors (just to make sure the fuckers stay in place).
Solder another two wires for Left and Right RCA. Be sure that they are long enough to reach L and R pins near the volume knob, but not too long because they are gonna be a pain in the ass to tidy up.
Step four:
Solder all hanging wires. Left RCA to the second pin (from the top), Right RCA to the third pin, and the ground to any place you're happy with. I soldered mine to the first pin. Go easy on the amount of solder, and be careful not to drop any solder on nearby PCB elements.
Step five:
This took me ages. Tidy up all your wires, and put the back cover back in the place. I had to file down a little + plastic bit near the battery connector in order to place wires underneath it. Also the place where the middle screw goes will be touching the wires, so you need to position them around it and somehow keep them in place. Or you can find another way of making it look neat and share with me!
Have fun modding your consoles everybody!