Hints on how to attach resistors to SMD LEDs
Unlike incandescent light bulbs, LED do not have as much thermal mass.  They respond instantly to changes in voltage.  Adding a resistor might reduce the voltage across the LED, but it will not prevent voltage fluctuations from reaching the LED.  To prevent voltage fluctuations from reaching the LED, a voltage regulator is necessary.

Consider the possibility that the car's voltage regulation system is not function as expected.

High powered LEDs do generate heat.  But this is unwanted heat which can damage the LED.  In such cases, measures are taken to quickly remove heat from high powered LEDs.