I've noticed that my classic impreza rear n/s passanger seat belt is wet in the retractor part, carpets aren't wet only had the car few week now anyone any ideas they'd be appreciated

 

Check the boot seals, the rear window seal and the spoiler seals.

Also pull the trim away in the boot and feel right down the sides for damp, that's often where it collects as well as in the wheel-well.
Another suspect is the rear lights, but my guess would be boot seal or rear window seal.