A car is a pretty noisy environment. Therefore, your grounding strategy may depend on how you handle the grounds and power in each of your equipment. i.e. how well they are filtered from the rest of the car's electrical noise.
Ideally, you don't want ground return currents from other devices in the car taking a route through your equipment. Therefore, a single point of ground return from all your equipment would be best. Make it a large return path (relative to your equipment needs) for a good return path for the current. Then connect all your equipment to that one ground path. Also, be sure your equipment does not accidentally touch the car chassis and provide another return path inadvertently.