Assuming the cars are all independent of one another, i.e. any given car has probability p of breaking down, regardless of what happens to the other cars...
Then two cars breaking down is two independent events, so to find the probability of it happening, you multiply the probabilities; p x p = p^2 ... less than the probability of one car breaking down
However, (I think this is where you're getting confused) if a car has ALREADY broken down, the probability that another will break down is still p, because they are independent.
If they aren't independent you need to know how they are related in order to answer questions about them