Getting back to the original topic. If the LED use a basic resistor to limit current, a dimmer, either linear or pwm version. However if any of the LED use active current limiting like a switching regulator could be damaged or malfunction if used with a dimmer.
Long and short of it, if any of the LED bulbs have a circuit board with "chips" they're not going to dim well if at all.
My marker lights are fine but the tail lights have active circuitry to manage the LED brightness, switching between running light and brake light modes.
