The best grounding point for a car is the engine block, as well as an additional small ground water connected to the inside fender or other metal part of the car.
The engine block and the cars chassis are excellent electrical conductor and provide for a path of low resistance for current to flow back to your battery.
You should also have a grounding strap from the engine to the body to prevent electronics such as the ignition control module from burning out.
A 12V battery does need to be grounded or it will not work as the ground wire and the positive wire have to be connected so that it can complete the circuit.
However with a 12V battery you don't need an earth ground connection like you do with AC voltage.
If on your vehicle or anything else you should also add a second small ground wire from the battery terminals negative side to the metal part of the vehicle or other thing that you have the battery connected too.
The black wire on a car battery is the ground also known as the negative with the --- Symbol.
The red wire on the battery is the positive wire which has the + symbol.
The red wire for a car or truck or automotive battery goes to the positive side of the battery which is + and the back wire always goes to the negative side of the battery which is the ---- symbol.
If you connect the battery cables wrong the battery will spark and can catch fire and it can ruin your electronics in your car.
Also make sure that the battery cables are tightened down enough to make contact and prevent starting problems and charging problems.
And make sure the battery cable ends and battery terminals are also clean and free of corrosion.
You can clean battery terminals of the corrosion by mixing some baking soda with some water and then poor it on the battery cables and allow it to sit for awhile.
Then brush the battery terminals until clean.
You can also dip the battery cables ends into some baking soda and water and allow it to sit and then brush off the battery cable ends and rinse with a bit of water.
When your battery cable ends and battery terminals are corroded it can prevent the current from flowing through the battery cables and prevent the battery from charging properly are cause a clicking sound when you try to start the engine.
If loose enough the engine may not want to start at all.