If you have unlocked the Gold Standard Kart and Gold Glider in Mario Kart 8 Deluxe, there is one more vehicle customisation part left to complete the prized set.
That part is the shiny Gold Tires, which are unlocked if you are able to beat the Staff Ghost on every course in Time Trial mode. That’s no mean feat either, meaning that this will likely be the last part that you unlock in the game.