Hey Paul.. Thanks for your post so many years ago.. I scoured the internet for days before finding your post..I actually contacted Vauxhall about this issue and after several emails between us, they came back with the following:
"Further to your query reference to the brake lights in your vehicle, I can confirm that our Technical Department have examined a like for like vehicle and can assure you that your vehicle is operating correctly.
The brake light is the top bulb and the one below, which is a twin filament is a side light only. The reason it has a twin filament bulb is because other markets or countries may use it; however, on a UK vehicle, there is only 1 set of terminals in the bulb holder so the second filament will never work.
I can appreciate that our owner's manual holds our old statement and I will ensure that this information is passed on to the relevant department for future ammendments."
Hope this helps
And OPEL Vauxhall really did everyone a disfavor with their brake light set up on the Corsa D..
When I see two brake lights lights that don't work I think fuse.. So I check every fuse in the car at least twice over the course of a week trying to figure out what is wrong.. I tested and replaced the 2 filament bulbs a few times and finally replaced the brake light switch... Replacing the brake light switch was a real torture to get it out and get it back in seated correctly..
Last night I disconnected the battery completely to let the car's computer module drain and reset..
So at 1 AM in the morning I read your post and knew right away that that was the problem.
I have a Vectra C and that car has no double filament bulbs for brake lights.. They are the standard 1 filament P21W bulbs, that when you turn on the head lights the bulb lights up like a running light but when you apply the brakes the same bulb lights up brighter..
So I changed the top single filament bulbs and everything works as expected..
OPEL screwed the community on this one in my opinion.