Water may be leaking through the plastic window seal. Check carefully to see if your plastic window may have been changed in the past. Some repairs are done very well and are hardly noticeable. If the window has been replaced then the seam between the clear plastic window and your top may not be watertight. If this is the case then you should be able to see water leaking from the bottom of the rear window.
Try spraying the rear window with some water then look back from the passenger compartment at the inside of the rear window. You may be able to see or feel if water is leaking