This trick works in 1 Player, Time Trial, or the Course Editor modes. To enable these cars, go to the Car Select menu. Move your cursor to "Garage A" then hold L1 and press Circle. Now move down to "Garage B" then hold L1 and press Circle again. To finish, move down to "Garage C" then hold L1 + R1 and press Circle once again.
Note: These cars are temporary and cannot be saved.