Hello Stefano,
I have only ever driven one 409 before but I am quite sure it didn't have that Tannoy switch (it doesn't look very original). I thought Tannoy only made speakers and PA systems?
Judging by the grilles in the rear parcel shelf, I can only assume that a previous owner was trying to emulate the rear cabin ventilation which was introduced with the 411 (the 411 has a small vent in each rear wing). However, I'm not sure where the air would be entering or leaving from the trunk/boot of a 409, or how it would reach the rear parcel shelf.
Maybe the fan was to provide a supply of air to the mice!
Cheers,
Kevin
PS. There was also a mouse nest under the rear seat of my 411 when I bought it, but thankfully the Antipodean mouse did not have a taste for Connolly leather
