go to...
double dash - http://www.gamefaqs.com/console/gamecube/game/516710.html
http://www.gamefaqs.com/console/gamecube/code/516710.html
http://cheatsguru.com/gamecube/mario_kart_double_dash/
ds - http://www.gamefaqs.com/portable/ds/game/920788.html
http://www.gamefaqs.com/portable/ds/code/920788.html
http://cheatsguru.com/nds/mario_kart_ds/