I just thought of what I have in the car right now that could be useful in such cases, and I must admit that I have nothing there that would have stopped the crime. I probably have some flimsy jack buried below the trunk's floor, but it would take an eternity to get to it. The deadliest thing I have that is readily available is a bottle of glass cleaner. Maybe keeping a Taser, like C2, handy is a good idea ... it's legal in CA but not legal in 8 other states and a few largest cities (where it would do most good, naturally, that's why it's not allowed there :-)
Bare hands works for me.