Hi there
I had a problem with re securing windows to their rails a while back and I was recommended stuff called PU18. It did a great job on these and also on the front locker rubber seals. It goes 'off' quite quickly so you don't have much time to settle the rubber, but it should be fine.
Regards
An old trick to find a leak was to use really black coffee,as water is clear and is hard to spot sometimes.Just stir it into a watering can of cold water and go round the seal.Any coffee thats getting in should show up a black run.