As a previous poster said, if the power drawn by the lights is not what is programmed then the blinkers flash fast and the car will tell you to check trailer lights. This can be caused by missing ground or by LED lights on the trailer. I have just bought a trailer with LEDs and it does this. I have also noticed what a previous poster remarked on and that is that even with the car off, parked etc the trailer lights flash briefly at around 10 second intervals. I assume this is part of the circuitry checking that lights are ok (?)

I want to know how to get the car to recognise that LED lights are working - preferably without just adding load resistors to present the 21W load like a normal light since that will mean if the LEDs do die then the car wont tell me - apart from the extra power being wasted.

cheers,
Seedar