You have a 'Mk1’ DS7, this is a known issue and I had both lamps replaced under warranty (I had extended one).
It should still be replaced free if charge as its faulty manufacture/design.
Issue is the sealant on top edge of lamp fails over mid 3/4. If you remove lamp cluster and invert you’ll see where water pours out (its doing that with boot open - just cannot see the edge)
It is possible to fix yourself, remove cluster from car, empty as much as possible then blow/suck cool air through lamp from around plug entry. Clean grove on top of cluster between black housing and clear lens. Then use bathroom clear/grey/black silicone to fill the groove all the way along top of light and smoothe off. Allow to dry
This is only possible if the reflective silvering in light hasnt started lifting…
Oddly, seems unknown in France but common in UK, (as is the pushed out rear suspension bushs -probably a result of UK road state).