Why is my car stereo working if it's not grounded?

I installed a new stereo and it seemed fine but when I started driving it began to turn off and on again. I began to check for loose connections when the ground wire seperated at the butt connector and the radio stayed on. I buried the cars ground wire and attached the parking safety wire ( it's supposed to prevent the DVD screen from coming on while the cars e brake is up) to the radios ground wire and every thing worked fine and stopped shutting off. I checked continuity between the chassis and the radios ground wire and got OL I checked DC volts between the hot and the radios ground wire and got nothing I don't understand how it's working I am confused.