One night, Darius slept inside Mariah. In a dream induced by the Bodach, he was surrounded by clowns inside the car. He begged Mariah to open the door but she said she couldn't do that; their destination was the circus. (TV: Dream-Eaters)
William Pike also owned a cab which appeared identical to Mariah, but was never identified as being the same cab in-narrative. The official K9 site, which this wiki doesn't consider a valid source, states that she was a converted ancient cab.