Many car manufacturers have tinkered with the charging system to increase fuel efficiency. Hence, the charging voltage will go up and down as the battery and other accessories need power. Cheap LED bulbs are made from unmatched sets of individual LEDs, which may not have the exact same voltage at which they turn on - this is sometimes more obvious with hot or cold weather. As the rows of LEDs are typically in series, if one doesn't turn on, the whole row is out. Better (i.e. more expensive, not Chinese) LED bulbs have a power supply circuit built-in that keeps the voltage and current constant, preventing flickering.
