I was bad enough at Rad Racer that I didn't even know it had an ending. But it's been brought to my attention that not only does it have an ending, but you can see it with almost no effort (which is one of the best kinds!).

So if you want to see it without actually racing, you first select a car

Then while at the 'Press Start to begin' screen

Grab Controller 1, press the B button 64 times(!), then hold Up + Right on the Cross Pad and press Start.

Done right, the ending will start

But be aware, there's not much more to it than the screenshot above.