Try posting this in the Audio section. Scott will definitely know what you need!

I would add a pair of 6.5" speakers in the rear deck. It can't be that hard to cut a pair of holes in there. At least 5.25" though.

Since you don't have openings in the left and right side of the dash (are there?), why not install speakers in the door and cover them with cloth of the same color as the interior. If you could do this you could do a pair of tweeters high up in the door and a pair of 6.5" in the bottom part of the door.

Don't do this if you want to maintain originality!

Good luck,

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2008 BMW 335i Coupe