This is a little homemade experiment I made.
A mono audio jack wire in the standard headphone size .. look in picture You can get it from a stereo
rock band foot pedal or similar thing with audio out port as connector
solder/soldering iron

Most of the pictures aren't my property but fellow guitar heroes here on Instructables. Likewise, most of my ideas are based off of theirs as well.

Step 1: Open Up Your Guitar

Take out all of the screws ..... open the case.... if it's wirelesss dont remove both sides of the case ntil you have room to put them next to each other (they're connected)

Step 2: Locate Where You Should Be

Here. Look at that big green chip in the middle.

Step 3: This Thing! What to Do

Ok, so you've found the chip, right? It's big, you cant miss it.
In the picture here you will notice that there are 4 solder points in the blue outiline.
these are on the nice big green chip. good. pick two of them either both rights or both lefts.

Step 4: Let's Solder!

Alright so take the two ends of the audio wire and solder them to the one pair of those points. One wire to one point, and another to the other point. You probably know how to solder if you own the gun, but if not make sure you figure it out before attempting this. Go by the DHYI rule DONT HURT YOURSELF IDIOT. Tape over it or hot glue it to secure it.

Step 5: Holey Guitar Hero!

Ok so now it's time to drill that hole! Thisis very straight forward, make sure that you can fit the jaack at the end of the wire through it, I recomend very close to the Select and Start Button. It can also go on the side, like bobabot's Hot glue the jack in there and close her up.

Step 6: Thats Just About It!

Now let her rip... it works like a charm for me