Almost every device that claims to allow cars with CANbus to work with LED trailer lights.*
For example:-
https://www.linearelectronicdesign.com/resources/LED-Pamphlet.pdfEuropean 4WD’s have an in-built diagnostic system that will cause the trailer LEDs to flash/pulse continuously and the vehicles will not recognise a trailer is connected.
A few European-made vehicles had that sort of thing at one time, but it is is far from universal
The adaptor just loads the car with a high power resistor in parallel with some lamps, resulting in surface temperatures up to about 120 °C. It also delays turn-on of the actual light slightly.
The actual trailer indicators are no longer detected with that adaptor in place. The blinkenlight on the dashboard will illuminate to show that the trailer indicators are working, whether or not they are.
*All cars have had CANbuses since about 2000. That has very little to do with electronics in cars that try to detect lamp filaments electronically, other than electronics make both possible.