Yeah I was kind of vague..What kind of system do you have stock? Is it standard or is it the premium? Do you plan on adding outboard amplifiers? Or do you just simply want to drop in speakers? Are you adding sub woofers?
I ask these questions because it's almost impossible to answer your original question without additional detailed information.
Good, now we can get down to specifics.
First thing, if you don't have a JL Audio Clean Sweep OEM integration unit, you will definitely want to pick one up ASAP (more specifically the CL441dsp) as the factory iDrive system and factory amplifier have preamp levels that will make it next to impossible to properly tune outboard amplifiers correctly.
Speaker locations on the wagon are in several areas. In the front you have a 6.5" mid driver in the kick panel by the feet followed by a 3" mid bass in the door along with a 1" tweeter. There are very few aftermarket component systems offering a 3 way system.
In the rear you have two speakers in the headliner above the passenger seats and slightly behind. I believe those are 5 1/4" mid drivers and the tweeters should be located in the rear doors.
I also believe there are two 6" sub woofers in the front under the passenger seats, those will need to be disconnected/removed as they will no longer be necessary if you plan to add component subs in the rear.