GBA Cart and Ds gba reader help

[Chi-kitory]

Not sure how to best describe it, but i know that the slot 2 of the ds can be used to read as a controller just like the Guitar hero ds. Can someone give me the parts of a gba cart so i can mod a controller into it.. anyother thing is can any gba cart be used as a controller?

IF you need me to clear this up some i dont mind... Sorry