Hi to everyone here - 1st post (just found this forum) but a long standing GTA owner (12+ years)
My GTA had the same problem (offside rear) - went into Renault Manchester for an extensive (and expensive) water test. The reckoned it was the door seals on the front door - a few hundred quid lighter to my disgust, the problem was still occuring. Why they thought it was the front seals was beyond me - but hey, you go with what the "experts" say.
Anyway, I decided I would dedicate a weekend to looking at the problem myself. After a few hours pulling apart the rear seat area, I actually found the source - it was the rear seatbelt fixiing on the main body. From memory I had to remove the rear seat but after that, access to the bolt fixing was straightforward. I actually saw water coming into the car via the fixing during a test - I reomoved the bolt and the fixing and then used some PTFE and some rubber sealent. 8+ years on not a drop has got into the car's rear footwell/seat area! Problem solved!!