I have often been irritated by drivers including myself, who leave the indicator on in the Sirion because they don't hear the little click it makes. (I am a Driving Instructor for the elderly)

I thought about amplifiers, buzzers from trailers and considered all the wiring options. Finally I have just fixed 2 auxiliary buzzers to the rear light cluster. Cost from Maplin £2-59 each. + 2 Scotch clips + double sided sticky tape to hold it.

I moved them from the cabin because a bit too loud, but I can muffle them if they get on my nerves. At least they will not get left on.

Simple and cheap solution to a dangerous problem.
I wasted time dismantling facia to find the wiring system and the fuses but got there in the end.