One possibility is clogged drainage vents on the bottom of the lift gate. One day, as I was shutting the lift gate on our 05, I heard the sound of sloshing water *inside* the lift gate. Turns out, there are about four small, black, plastic covers on the bottom of the lift gate that can be pried loose. These cover drain holes to let rain water out of the lift gate. Mine were clogged with dirt and grit. I pried loose two of them and water drained out. No more sloshing sound. I left two covers off to (hopefully) prevent future clogging.
Also check your rear side windows - my sister's '99 has water leaks from both which we are assuming is due to worn-out gaskets. You need to get inside the vehicle and have a helper direct water onto various areas with a hose until you can figure out where it's coming in.
One final possibility - from the bottom - if there are any leaks in the floorpan, you can get water spray when driving which will leak up in there as well.