You can add custom decals or symbols to your vehicles. You can have up to 7 "layers" per vehicle. Symbols, being made of multiple decals can represent several layers towards your total
You can also add custom parts to your vehicle. One tree of advancement in unlocking more parts is your Tuner Level. Tuner Levels are obtained by spending time in the vehicle customization terminal.
You may get custom grills, sideboards, spoilers, wheels, lights or other parts for your car by advancing your Tuner Level. Some parts remain locked however for other forms of advancement (via combat).