What you are describing is likely alternator whine and can be verified by alternating the rpm of the engine and listening for changes in the frequency of the whine. Verify that the ground is in fact grounded to the body and try to avoid going through the normal fuse boxes for the positive power. Also try winding a choke on both the positive and negative sides using #43 toroid material. Alternately, solder a .01 mF capacitor across the contacts of each speaker and this should also attenuate the noise you are hearing.