Love this album and song.

A “black mariah” refers either to a police wagon or a hearse. In the case of this poor fellow he’s got a wooden coat and he’s never coming home—the narrator is seeing the hearse coming, in the form of a big black Ford.

It gets a bit confusing when the Clash sings “no need for the black mariah” in Guns of Brixton—in that case they’re saying there’s no need for a police wagon, as the protagonist of the song has already been shot dead. Presumably they’ll need a hearse though.

Nick Cave uses the slang for a police car in the Curse of Milhaven, where the protagonist is “off to the asylum in an old black mariah”.