Ira
It will either be dim or possibly not be enough voltage to light it at all. LEDs run on 3-4v, It will already have a resistor in it somewhere to run at 12v.

Johannes
LEDs have no standard voltage! that can range from 1.6 to 4V. But you do have to limit the current through them. This why it is a bad idea to connect an LED directly across the battery terminals. Current is limited by a resistor normally. As yours is for a car, there should be a resistor suitable for 12V in series with the LED somewhere. If you reduce the voltage, the current will be reduced and hence the birghtness will be reduced.