There is no 300cc cup on Mario Kart Wii.
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Wiggler is not unlockable in Mario Kart Wii.
If you need to unlock her in Mario kart wii here is the answer: Win the 150cc Special Cup. If you need to unlock her in Mario Kart 7 here is the answer: Win the 150cc Mushroom Cup. Source: I beat Mario Kart Wii, and I unlock Daisy in Mario Kart 7.
there is no cheat for it
You can't.
I have Mario Kart Wii and played it lots of times. I don't think you could unlock more characters. Or if you could, IDK.