Pop the front bonnet, remove the plastic cover that sits on the battery, grab a flash light and have a look around the back and you will notice two drain holes one on each side. Sometimes they get blocked up with dirt and leaves. Grab a clothes hanger put some tape on the end of it so it doesn't damage anything and poke it down the holes clearing all the dirt and grime out.

Well, after a few months of using this, I can't recommend it. The wire mesh is very fine and gets clogged with dirt and the water doesn't drain. Got a little water behind the driver seat today. Live and learn.

It might be a good idea to familiarize yourself with all the drain locations on the car. Start front to back, in the front bonnet and work your way to the back. Sounds as if the rears are clogged. Put the top half way down and you will clearly see the drain holes. The important thing to be careful of here is the computer under the drivers seat. If it gets wet, and shorts out, it is very expensive to replace. This is actually how I got my Boxster, the previous owner had this happen to him. I got the car with a new computer, then had that replaced as well when Porsche couldn't code the keys to the new computer.