The diode method above gives the bulb half wave. So it sees power 50% less time per second (half an ac wave) and is cooler because of it. Some folks might see that pulse rate as flicker though
The evening out of the waves done by additional diodes in other cars as mentioned. But that means keeping track of which cars might be run. However, two diodes back to back ,opposite bands, lowers the ac in both directions of ac by about 0.75v. Two pair inline, about 1.5v ...etc etc. But that also means a dimmer light. Cheaper and easier to change bulbs than either diode method imo. But it will run cooler and be bright with halfwave; just maybe a little flickery. (can add a cap to smooth that, but again bulbs are cheaper& easier imo... In fact, you have to use diodes for LED on ac; so might as well go led at that point.
