Ours has "water" in the boot that comes from condensation dripping off the boot lid overnight. Buy a cheap silica bag off t'interweb and see how you get on before trying the (very, very annoying) job of replacing the seal. We did the silicon reseal of the brake light and then found out it was condensation...
The gasket is over £30 and on mine didn't even work, I ended up making one myself out of a 5mm rubber draught sealer.
It was only sticky on one side, I had to trim it, but it worked and was much cheaper than the original which came much to thin. The foam I used was 5mm but 3 or 4mm would be fine.
