Tamer below is a link to photo on xkedata.com that might help. Looks like there are a lot of dash wiring photos of car 875423 there. I have never tried to remove the light switch but I might suspect the grommet that surrounds the switch plunger might pry or screw away from the body to withdraw the switch from the car.....but I am just guessing.
http://www.xkedata.com/gallery/zoom/?id=82082
David Barnes
68 E-Type FHC