You need a power transistor which acts as an increasing resistance between the battery and the lamp to slowly decrease the current through the lamp, or a transistor which is pulsed on/off at a rate above which your eye perceives it (flicker fusion) such that the ratio of ON time to OFF time slowly changes to gradually decrease the average current through the lamp (Pulse Width Modulation)
Google the colored terms. The first method dissipates a few watts of power, so the transistor has to be mounted on the car body to act as a heatsink. The second method dissipates less, but the circuitry is much more complicated.