you could put a linkage under the dash to operate a toggle switch from the stock knob. I did this on my truck to run a home made 12v wiper setup from the original vacuum wiper switch. There's some pictures
here of the setup at the bottom of the page.
You've got a great webpage of interesting pics Giles. I have great respect for you guys working on American vehicles and having parts so far away. It sure helps to have some engineering/machinist skills.
I worked 2 years in Bromley and do miss the beauty of Southern England. Cheers...
