Check your tail light gaskets. . . .
 

if you have water in your trunk. I was getting a nice pool in the drivers side well after every substantial rainfall. A few weeks ago I checked the gasket and it looked fine but I could tell water was getting passed it. I cleaned the surface that it presses against and torqued the you know what out of the nuts and I was relieved to find a dry trunk well this morning after a heavy rain last night.

The new gaskets are a b*tch to install. Older gaskets will have the metal frame press into them, creating a good seal. The new gasket is still pretty round, requiring more force to install properly.

Don't disagree that the taillight gaskets can let water in when they get old and dry or deformed.

But just a note that you're in beautiful California... as a caution to those in more rusty states (or provinces_, you won't want to "torque the you know what" as you'll be more likely to hear a snap as the whole thing lets go.

Gently snugging them up will work better :D