To get back on topic, the only cheats that I have heard of require the GameShark disc to implement. But FWIW, the game is much more rewarding to work through without the cheats.
It took me a fair bit of time to buy the Mazda 787B, Audi R8, Pescarolo C60 and Toyota GT-One, but when you get to that point, the first run down the Mulsanne straight at Le Mans is ample reward.

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