The Mechanic's Creeper is a tool available to use after the player has constructed the Garage. It's used to gain access to the underside of the Konig for repairs, upgrades, and maintenance. It is especially useful for swapping out the car's transmissions, differentials, or exhaust systems.
A second Creeper can be found at the Speedway, in the same garage as the Racecar; and a third is at Antoine's house, beside the Prinz.