So I'm planning to replace my front and rear door speakers. I understand that its 6x9's in the front, and 6.5" (or 6.75") in the rear doors. I look at crutchfield to get an idea of which models/brands, and they filter out most of the speakers in these sizes that they sell. Am I missing something... are not all 6x9's the same - is there some clearance issues I need to watch out for? Someone enlighten me.