bert wrote:

some time ago you were required to have an indication that your trailer
indicators were working. Originally that was visible and still is on my
Defender but then switched to audible, to save the manufacturers putting
in a cable and dash light which in most cars was never used. Don't know
what it is now

My car has a a single extra "trailer indicator" lamp on the dash, and if
there is a fault, it displays a message to the effect that trailer
lights aren't working ... not sure if the circuitry for that already
exists, or was fitted as part of the towbar kit.