That foam is a be-atch to take out. You have to remove the two upper rear inside trim pieces, the two lower inside trim peices, and then the lower center rear trim to get those foam inserts out. I literally cut the thing (right one) in four pieces VERY CAREFULLY to get to the central locking pump module. Anyway, the two right and left upper rear trim plus the lower center rear trim at the lock, then the two (left and right) real inner panels (lower) will give you the access to remove those two foam inserts.

I was able to slide the foam insert over a little before taking the drastic measures to cut the right one up though. I didn't see any water then but wasn't satisfied there was or wasn't any in there.

You can also pop a little hole into the rubber plug on each side accessable from the bottom of the car. They are positioned near the rear differential on the bottom of both compartments. Just get under the rear, you'll see both the plugs protruding. I would just stick something in there and pop a hole into it, then plug that hole with something you have. The plugs are thin so pushing something into it should be easy. You also might be able to use a single edge razor and slice a line into/through these plugs, if there is water there it should run out. Sealing the slice may or may not be neccessary, use your own judgement. I didn't pop the hole into it, as you see above I decided to tear into it from the top and hacked the foam into 4 (or was it 5) peices.