It is a way of calling the car bomb ("vehicle loaded with explosives"), separating both words and in places where the car is commonly called a car. Actually the autobomba ("mobile hydrant of the firefighters") would be a confusion, since the prefix auto- is for "autonomous pump" and not for the vehicle; that's why it's a feminine noun .
CAR PUMP cistern vehicle used by fire brigades to put out fires, which has its own water pond and propellant pump The term has been made generic for all fire trucks although sometimes distinctions are made, such as ladders car, for example.