OK, I am a EE, but converted to IT Project Management about 20 years ago. The challenge with working with LEDs is that most folks understand voltage - i.e. higher voltage, brighter light. However, the LED is a light emitting diode, which is a current device. It is made of a little piece of silicon with a doping material, like gallium added. When a current passes through it, it emits photons, or light. Put a bunch of them together and you get a tail light. They almost always include, in the housing, a current limiting device, often a resistor, to prevent too much current flowing in the event of a short, water, etc. Raising or lower the voltage doesn't affect these, unless by doing so you reduce the current below the amount needed to activate the diodes. I suspect, but don't know, that the PCM is looking for a voltage drop when the light is on, because it was designed around 12VDC incandescent lamps. The manufacturers of the replacement LED tail lights must put in circuitry to fool the PCM into thinking the voltage is dropping when the light is on. One of the issues is how little current is actually needed to activate the LED. A single LED lights brightly at 20mA. That's 20/1000 of one amp. If there are 50 LEDs in the housing, then 50 x 20 is 1000, so the total current is 1000/1000 A or 1 A. That's about half of what a single filament 12VDC lamp draws. Also, about 85% of the energy that goes into an incandescent lamp is released as heat, shortening the life time of the bulb. LEDs run very cool and nearly 100% of the energy is light.
Thanks for taking me back to 1978 when we were just starting to use LEDs in electronics equipment and nobody knew how to make them work correctly!