You want VAG part 113 954 515H. It looks almost the same as the one on your picture, but has a tapering M10x1 thread.
http://www.partsbase.net/part/aud-113945515h
I have a problem with mine though. Looks like it's a bit too sensitive, and it does not always disengage when I release the pedal. Looks like the residual pressure due to friction in the MC keeps it closed. It switches of with time, espessially if I tap the MC with a wrench. Might be due to the fact that I connected mine to the secondary/front circuit, not the primary/rear. Or mine might just be defective. Ideas, anyone?
Update for posterity: I did notice that in the book Chris specifically mentioned the pressure switch has to be plumbed to the primary circuit. I rearranged the tubing a bit, now the brake light works flawlessly!