Answer:
Try pressing R1, L1, UP, RIGHT, LEFT, DOWN, L3, R3, X, SQUARE, CIRCLE, TRIANGLE then go save the game. After this, go to the Mission Select screen and select a mission where you see Snoop Dogg so you can play as him. This is just one of many, many cheats available for Streets of LA for the PS2. To find more cheats (ones that you would actually want to use) go to ign.com for cheats to all (or almost all) current video games.