She could communicate in a feminine voice, operate under a voice recognition system and move without the presence of a Human driver. She appreciated Darius clearing out junk food wrappers from the back seat.
In 1963, Mariah was originally owned by Darius' great-grandfather William Pike. She was later modernised to run on alternative technology and given a cybernetic brain. (K9TV: The Cambridge Spy)
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. 
One night, Darius slept inside Mariah. 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: Dream-Eaters)