Unless I'm mistaken, this glitch was the Dupilication Glitch. You had to press both the Y and X buttons at the exact same time and you would gain an extra part. You could only take these vehicles into the Test-O-Track though.
This glitch was also patched. It wont work anymore.
U could probably do it if u empty ur hard drive cache, but i've never tried it...
Plus it wouldnt work online.....