There was a post last year with the same problem. The poster has come back to let us know how it was fixed. http://www.citroen-o...2009-c5-tourer/
I replaced one on our 2007 C5 estate since water was getting in and whilst trying to remove the lens to dry it out and seal it the lens broke. Whilst leds may last a long time, the printed circuit may not if it gets water in.
It is not a quick job to replace on the tourer, this is based on the Citroen procedures so should help ( I have not done the job so maybe you can report back with tips after you have done it):
- put a sheet or similar on the rear of the roof to avoid paint scratches
- open the tailgate and remove the top tailgate plastic trim (8 clips)
- undo 4 torx bolts now seen holding the spoiler on (4 Nm)
- rotate the back edge of the spoiler up to unclip 2 hinge pins
- unclip 2 clips nearer the middle of the spoiler
- uncouple the wash water pipe (it may need an insert pushing in to release it as on fuel filter pipes)
- lift off the spoiler and undo the light electrical connector
- unclip the stop lamp and remove 2 bolts to release it from the spoiler
Check the new light works before fitting everything. The procedure says to disconnect the battery first but I did not bother on our C5.
If you do not want to buy a new light to start with, then you could check for 12 volts at the connector when the brakes are applied.