I've got this problem too - I should have checked it out with you guys first I suppose!
Over the last few months (during this crap weather) I've had at times over two inches of water stood in the rear footwell.
When I've set off after a heavy downpour the rainwater floods through from behind the glovebox and into the passenger footwell - eventually making it's way into the rear. There is never any damp inside the glovebox however.
Last week I had Autoglass fit a new screen (thinking that the old one was fitted quite poorly) and I also fitted a new pollen filter just to be sure.
Well, the rain really came down this morning here in Somerset - the first time it's been so heavy in a couple of weeks - set off to take the kids to school this morning and what do you know - water piling in from behind the glovebox - back to the drawingboard and back to bailing out.
I will certainly take the advice above and let you know if I've had any success.
All the best,
Steve