Bears break into cars to get food because they smell food. They don't care whether convertible or not, they do what they've learned, which is usually ripping open the doors. This happens in the places where bears have learned that cars are food containers, Yosemite is a particular problem because humans haven't learned there.
They are not likely to attack occupied vehicles of any type, or moving vehicles. I think we have managed to move past the days when tourists fed bears from their car windows.
However, in a territorial battle of motor vehicle vs bison or moose (or a domestic bull) I would bet on the animal vs a Miata. When I was driving a little 1600 BMW, and a large ungulate was in possesion of the roadway, I would yield. Now driving a 6000 pound truck, in my mind the bison, bull, or grizzly bear still has right of way.