Choose Downtown. After the windmills and curves there is a tunnel. Right before the tunnel there is a tree - head for it, there is a gap in the fence that will lead you under the waterfall and right near the exit.
Select your vehicle and track. Wait for the countdown and when it displays 1 hit the acceleration button quickly, then let go. For this to work correctly, make sure you are not holding the acceleration button when the screen says 'go.'
First Secret Car:
At the title screen where it says MRC press R. You will see a time you must beat the drone in "Match Race". Choose "Match Race", select any track, then select a car. If you win all three races, then you can use the KINGROADER. This car is mainly for off roading.
Second Secret Car:
At the title screen where it says MRC press R. You will see a time you must beat the drone in "Match Race". This time you will see the next secret car. Select any track then select a car. If you win all three races, then you can use the next secret car.
You can also activate the second secret car in two player mode. Go to the closed garage and press B. This will open the garage. You can exit two player mode and go to a normal race. Don't select a new car. You will already have the secret car selected...
Thanks to Revolution readers Blarkin and Jordan Jr., !