I've just fixed mine. It can be taken apart. Looky here http://homepage.ntlworld.com/igcrawford/vauxhall_corsa_automatic_selecto.htm
Haynes manual tells how to get it off.
You can then seperate the two halves of the switch (six screws) and clean out all the old dried up grease.
Be carefull not to drop anything
Then reassemble and the light wont appear again for another 10 years hopefully.
The tricky part is getting at the wiring connector below the battery.
The rest is easy when you've got it on the bench in front of you.
Hope this helps.
If your reversing lights are also working intermittently then this
switch is most likely your problem. I had flashing warning light and
reverse light problems.