Not sure what year the mark 3 is, but on my astra, with the rotary light switch on the dash to the right of the wheel, you just lever the complete housing out with a small screwdriver.
Get yourself a Haynes workshop manual, or see if your local library has a copy, there's a guide with pictures in there.
gibby
January 2010
Do you mean BULB,if so what bulb,SWITCHES are usually only found on door/boot openings.
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Try fixfault.com to fix cars.