Car Tail light - why the extra components?
It is likely a zener to keep the LED at a constant brightness.  Car power is not constant, it's only loosely regulated, and since the LED has a very fast response time changes in vehicle voltage (say when you start your airconditioning) may be percieved as flashing your light.

Filament light bulbs take awhile (100-300mS) to visibly react to a change in voltage, so it's not an issue for them.