She could communicate in a feminine voice, operated under a voice recognition system and could move without the presence of a Human driver. She appreciated Darius clearing out junk food wrappers from the back seat.
William's cab was never identified as Mariah but as it appeared identical to her and the official K9 site states that she was a converted ancient cab, it is likely it was. 
When Drake tried to turn Darius to his side when he framed K9, he had K9's duplicate severely damage Mariah at night. This made Darius furious, but when a CCPC grabbed Jorjie he stayed on K9's side. (K9TV: Jaws of Orthrus)
One night, Darius slept inside Mariah, there a dream induced by the Bodach had him being surrounded by Clowns inside the car. He begged Mariah to open the door but she simply said she couldn't do that and that their destination was the circus. (K9TV: Dreameaters)