I had a similar problem with my Mondeo a few years ago. Turned out there was a water leak into the boot, which collected in the wheel well. When cold not much happened, but when driving (and the car was warmer) some of the the water 'evaporated' and condensed on the cooler rear window.
I never really found where the water was getting in to the boot area, so I drilled a small hole in the lowest point, stuck a 25mm length of plastic tubing into hole so water would not come in, but would drain out...... no problems from then on.......
Have a look in the boot/wheel well. I read somehwere that there can be leaks in these cars.
