Just because you have the battery grounded to the engine does not mean that the engine is grounded to the body. The engine is mounted in rubber insulators and so is the transmission. The driveshaft which may or may not make contact through the oil in the tailshaft goes to the rear end which is also mounted to the springs which in turn are insulated from the body. So grounding the battery to the engine is not grounding the battery to the rest of the car or to the ignition power sources. The battery power is routed to the dash and then fed back to the rest of the electrical system. Therefore the body rounds to complete the circuit. Hope this makes sense to you. It is hard for me to explain in print.