A little more info about grounding...
It's not always a good idea to ground directly to the battery, as long lengths of wire can be the cause of ground loops as well, as previously stated. If you make sure you have a heavy grounding wire (4gauge or larger) from the - terminal of the battery to the vehicle's body, then most often grounding to the body of the car will be fine. However, there will be noisier ground spots on certain locations of the body than others, and these locations are different for each and every car.
Also, grounding everything to the same ground point may not be necessary, and may even make the situation worse, depending upon the gauge of wire used and if the 1 ground point you choose was already a noisy location because of all the other ground currents flowing around your vehicle.
The BEST thing to do is try each and every suggestion you can find and see which works best for your particular vehicle/setup. (Or at least try them until you find one that you can live with. :)