I have a 2001 Alero coupe, recently started to see water just on the rear seat floor, both sides but not at the same time. Always happens after strong rain fall. I can't figuer out where it is coming from. I do have a sun roof, but the headliner is dry. All the windows were closed. The seal around the doors look okay. Any suggestion?

Do you have a substantial amount of rust on the underside??