I have the same type of problem with one of my R33's.
The boot leaks on one side at a time and fills one tail light. It is an intermittent problem that I cannot replicate by hosing the car and the tail lights have been out and resealed. Depending on which way the car is parked the water gets into the lower tail light,
The car has a factory GTR bootlid and spoiler and I am starting to think that it is leaking through the spoiler mounting bolt holes.
Any ideas or suggestions on what to try next would be appreciated.