I may be preaching to the converted here,but as you may have read in a privious post, I strongly advise against using the factory rubber mat. It traps moisture under it and promotes rusting.
Don't agree with this statement, Witt.
There is no general "need" to be moisture below the trunk mat per se. Hans ans John are right.
If you take proper care of the seals there shouldn't be moisture underneath.
However, for a daily user used heavily during rain etc. seasons you will be surprised where the water (moisture) will come from even if the seals are all correct.
But nowadays for our well-cared cars there shouldn't be a problem with that anymore. If you have/had rust issues there, remove this thoroughly, get the trunk floor correctly primed and painted. After a while wax the floor with body wax and that's it. It will keep like that if you don't use your car during rain too often or just flood it while washing.
John is fully correct about the trunk cocomat.
Use it on top of the rubber mat - to protect the original mat.
Best,
Achim