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.

1950 Chevrolet 3100 (occasionally working) truck:https://c2.staticflickr.com/6/5490/30684982201_6dc479d18d_c.jpg
1928 Chevrolet AB National Phaeton: https://c4.staticflickr.com/6/5573/30137792083_42886d30a4_c.jpg
1932 Austin Special Sports: https://c8.staticflickr.com/6/5793/30116567183_1520e67c4d_z.jpg