I have the front set of a/d/s x-overs in the trunk. The rear (side) x-overs are mounted to the inside of the body of the car via high strength adhesive velcro. The distance from the speaker to x-over is not critical, however, make sure to stay away from power lines.
It also depends on how you're wiring ur speakers. If you are tapping into the existing speaker wires, than it would be easier to place them in the trunk (since that's where the source of the wires are).
If you are running new speaker wires, it's easier to put them in the doors since this would only require you to squeeze one wire pair into the door instead of two.
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