Is it likely that a given car stereo will work in a given car? Are there a number of different form factors (sizes)? Are some more common than others? Are there a variety of connector schemes?
I'm thinking of the stereo itself, rather than the speakers. Presumably I would want to make sure the speakers are not going to be blown by a much more powerful stereo. Are there other compatibility issues between stereos and speakers?
What other factors need to be taken into consideration?