Torn seal around aperture.
Cracked rubber between glass and metalwork.
Parking on steep slope causes pooling leading to water breaching loose seal.
Pipe disconnected from rear screen washer.
If sunroof fitted, blocked drainage tube causing water to track back along inside of roof and down C-pillars. (check roof lining edges)
Bad repair to wheelarch damage/corrosion leading to rear wings filling with water and capillary action up into boot carpet.
Neighbour's Mastiff cocked leg up air vent
i had this problem on a ford, just damp on left and right hand sides of fitted carpet, about 6 inches in from rear lights, tried the lot/ first resealed back lights/ then put new rubber seal around the boot/ . then put sealer in and around back window/ had pressure washer on it , hose pipe tried the lot, in the end -
it was the small welded joints on both sides of car body inside the gulley just down from rear window.
a bit of sealer cured it . hope this helps. r.r.