Hi Krissy.....welcome to the Forum
It may possibly be coming through the rear light unit as there is a gasket between the light assembly and the body which hardens up over time and becomes less effective at holding out the water (especially if it is channeled from the tailgate down to the bumper).
Worth removing the light units and applying some silicon if you find thats where its coming through.
The easiest way to find where the water is getting in is to use a smoke generator in the car and see where it comes out of.
A good independent garage should have one to use on the car and only takes a few minutes to carry out the task.
Also, as routerspouter has mentioned, it could be something as simple as a rubber bung that it missing.
Let us know how you get on with it