All the switch on the light itself does is use a permanent earth, instead of the door activated earth.
Doing what you've done, you've established the light is fine, so is the live to it, so the issue is the earth given by the door switches, they are a simple wire, joined as 4 that simply go up to the light, the actual earth for the switch comes from the screw that holds the switch in the body, I can't see all 4 being duff, this leaves the most likely thing as a break in the wire.
BUT first I would check for a earth on the door side at the light switch, the actual switch on the light suffers terribly from corrosion, so use a test light to see if the doors do really switch and it is just gunge on the switch contacts, these can be easily cleaned, but the best stuff is DE-OXIT, this removes 100% of the corrosion and forms a permanent protective coating, it's well worth getting, can also be used on the door switches contact faces, I did mine around 6 years ago on the astra, 5 on the cossie and haven't had any issue since.
So check the actual light switch first as the doors may be switching, but corrosion is stopping it.
tabetha