Without any dedicated buck converter/white LED driver IC, you can safely drive many standard Hi-efficieny white LED modules using the battery power available in automobiles. Here is a safe and simple white LED driver designed for 12V automobiles.
In the Automobile White LED Light circuit, fixed voltage regulator IC1 (7805) provides a steady voltage of 5V across C2. Resistors R1 limits the current flow through the white LED D1 (3v6/350mA) with the help of transistor T1 (and T2), ie components R1, T1 (and T2) provide a constant current to D1. Use a good heat sink for T1. This LED unit gives a constant light output for input voltages ranging from 8 to 18 volts!