This 'Tannoy'-marked switch does not control the speaker balance, as an audiophile would suppose (I love Tannoy speakers and my main home audio system uses 2 Tannoy Gold Monitor full-bandwidth speakers is a Swiss cusotm-made corner enclosure).  It has five positions, 'Off', 'In' and 'Out', either 'Slow' or 'Fast', and this mean that a small fan in the trunk can move some air through a grille in the passenger's side of the rear parcel shelf, if required, either in or out to help the air flow in the cockpit

This is what we found removing the passenger's side panel in the trunk. At the top, nearly invisible when you open the trunk, there is this small fan that forces some air in or out of the cockpit  and is controlled by the small Tannoy switch under the dashboard. It worked, but it was not looking good

 
The panel holding the small grille covering the fan was quite rough.
it was advisable to restore the whole lot, but to do this we had to remove the back seat, and what we found there....

...was a mouse nest! During the years of non-use a mouse had built his nest under the seat.....

....and his need for food made him inflict a considerable, if scarcely visible, damage to the back seat leather!

The back parcel shelf, restored, grilles rechromed

The rear right trunk panel, looking as new, ready to be refitted

Looking very nice now, and the fan works