From the same Wiki article.

Self Variable Resistors
Some ballast resistors have the property of increasing in resistance as current through them increases, and decreasing in resistance as current decreases. Physically, some such devices are often built quite like incandescent lamps. Like the tungsten filament of an ordinary incandescent lamp, if current increases, the ballast resistor gets hotter, its resistance goes up, and its voltage drop increases. If current decreases, the ballast resistor gets colder, its resistance drops, and the voltage drop decreases. Therefore the ballast resistor reduces variations in current, despite variations in applied voltage or changes in the rest of an electric circuit. These devices are sometimes termed barretters.

I only stated that there were fixed and variable ballast resistors, and I speculated which were used in autos. Even if the ones in cars are fixed, there are still (self) variable ballast resistors, which regulate current themselves without turning a knob.