Sounds very like an intermittent problem I have, from rear left. I'll try the tape solution. At first I thought it was where the shelf edgs passes over the plastic housing for the rear window wiper, but that's more towards the right of the car. The parcel shelf is bad news anyway - I hate having to refit the press-studs every time I've had the back seats down. A suitable case for Velcro, like the Yaris!