No, depending on the way you have it setup, the cable should provide ground back to the head unit which would then be grounded back to the car.. however if its sparking then it means that the case for the phatbox was becoming the ground for the phatbox, and possibly the head unit, which could be very bad.. so by securing the unit properly it shouldn't change grounding points back and forth.
With a car, there isn't really and earth type ground, it just feeds back to the negative batt terminal via the car body...