Follow the track until it starts heading uphill and toward the left. In the distance toward the right there should be a "T" shaped wall. Use a mushroom right before you normally turn left, and jump to the right. You should land over the wall in the area right before the tunnel.
Mario Kart 64 Shortcut Video Walkthrough Series for Nintendo 64
Mario Kart 64 - Mario Raceway Shortcut Video
