[SwissXC90]

If LED lights are glowing dimly, the it is caused be the module in the car that provides the power for the lights not switching off completely.
In the industry we call this "leakage".
The residual "leakage" voltage is enough to illuminate the LEDs dimly.

You can solve the issue by adding extra resistive load to the circuit to provide enough load to "consume" the leakage and drop the leakage voltage down to 0, or so close it doesn't matter.

This can be done really easily by just adding a resistor across the LED lamp to provide more of a load.

It doesn't need to be a high power low-ohm resistor.

In many cases you can get away with a 1kOhm 1/4 watt resistor.
Costs around 2p, ie: nothing.
Fit the resistor where-ever you want in the trailer or in the car.
Ideally in the car, as it is the car that in at fault, not the LED lamps.
You could even connect it inside the trailer plug if you have enough space and it is weather proof enough.

Depends whether you will use the caravan on multiple cars (then fit to caravan), or use the car with multiple trailers (then fit to car)
Or fit to both.

You only need one resistor per circuit. not one per LED lamp.


The root cause is actually both car and caravan:

The car expects normal incandescent lamps on the trailer, and is not designed to be used with LED tail lamps

The caravan (trailer) expects the car to completely shut off the voltage to the trailer lamps when the lamps are not in use, and is not designed for use with cars that have a residual "leakage" voltage.

So point the blame finger where-ever you want. It'll get you no-where.
The solution remains very simple, regardless of the blame.