If it was sunny, the air in the ones with sunroofs would have been warmer than the ones without. The higher temperature differential between the air inside and outside would cause more misting.

Most of the cars in the street were misted up yesterday. A very mild wet evening, warm moist air, followed by a sunny day, then rapid cooling into the afternoon. The warm moister air inside the car will condense on the cooler glass surfaces, exactly as Adam or says. Both our cars were misted up, both dry as a bone inside, no leaks .

"e.g. tennis racket string or bicycle brake wire."

And as is often the case, prevention is better than cure. On an old Volvo we used to have years ago I would routinely poke a bicycle brake wire down the channels to make sure they were clear. I did this at least once a year, usually before the winter. A small jug of water would then determine whether they were clear, it used to drain out in the wheel arch I think.

<< In the past I've used strong firm thin flexible cable e.g. tennis racket string or bicycle brake wire. >>

I have used a length of bendy curtain wire - without the hooked ends, of course ...