right to the metal on the radiator support, always best to keep the ground as short as possible, then use fuses and relays to the battery for power and use the headlight harness positive as the trigger, then you will not have any problems at all.
***I recommend using waterproof relays and position the ballast so the harness is facing down, that way if it gets wet water will not sit in/on the harness and potentially damage the ballast