Never ground to the car's frame--if it's a car that has a frame. IN all cases, the sheetmetal is the best place, and this is where the alternator is grounded. THe body (in a car that has a frame) is isolated by rubber bushings and the engine is also isolated by its mounts. There is usually a connection between the alternator and the engine block (often the bracket). There is also usually a connection between the battery negative or the alternator and the sheet metal of the car. Those connections complete the circuit. Ground to a bare metal point on the sheet metal chassis of the car. Scrape away the paint and use a locking star-type washer. Don't use an existing seat bolt or other similar poiints. Those are often just spot-welded to the chassis.