Yeah I was going to say the same. My dad kept our CJ in the garage for 9 months out of the year. It was a doors off summer cruiser. It rained a lot where we lived, so why fuss with water getting in it and the plastic windows freezing and cracking during the winter months?
There could be an actual design defect of the Bronco, don't know and not disputing that. BUT common sense to me would be keep it in a garage during the worst of winter or opt for a hardtop. Or just assume that ANY convertible is subject to wind and moisture intrusion and prepare accordingly.