Cabrio ride with child (2J child seat rear)

Hello, I am currently thinking about buying a convertible (BMW 3 Series E36, E46 or Mercedes CLK zw BJ2000-2003). Since I have a son who will be driving with me, I would have liked to know if this generally works well with child seat in the back? The child is sitting a little higher with it. Would of course drive all windows closed and since I will use for the excursions, never use faster than 80km/h with child. Front passenger with disabled airbag or rear seat positioned does not come at all i n Question! What are the experiences for this? BG