Hello and thank you for using JA:
What I can recommend you to do is the following:
Lift the rear of the car and put it on Jack stand.
remove both rear wheel, check around the rear fender if there is any rusted area that could possibly the access of the water going in, if not, look under the car and see if you can find any hole, that is possible for the water to get in, most likely car lost one of the drain rubber seal.