Update with steps for replacing rear speakers:
First, pry up the grills with a thin edge of your trim tool. I think it's a little easier to work on the speakers if you put the rear seats down, although these photos have them up. Live and learn.
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I've got the HK so I've got tweeters, I believe the base stereo doesn't but here's a photo of the stock HK setup for the history books.
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Each speaker is held in place with three torx bits, same bit as the front speakers. You will need a low profile driver, but a stubby torx driver might fit.
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You can then pull up the speaker to find two sets of wires if you have the HK. The ones with the black heatshrink bundle go to the tweeter, and can be tucked back in out of the way unless you're replacing with a component set. I'm using Coax JL's, so I'll just leave the old tweeters there. The wires with the cloth tape are the important ones coming from the amp.
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I used a pick to disconnect the latch. A small screwdriver would work fine too.
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If you're using JL speakers from Selmeralto's post, you'll need to break two tabs off one side.
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Closeup of the electrical connections. Pay attention to polarity, it matters.
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Good crimpers are like sliced bread. Once you use them, you'll never figure out how you got along without them.
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I think I'm ready to rock here. I picked up the wiring harnesses from ebay, search for BMW OEM speaker connector harness.
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You know when I mentioned polarity matters? Well, apparently you can't trust BMW to be consistent. I connected everything up like I *thought* it ought to work, but I was less than happy about the bass response from the rear. Well, it turns out the left and right factory harnesses have the + in opposite spots. F**** f**** f****. Lessons learned - don't just trust the way the plugs fit into the socket, confirm the wires on one side of the plug match the wires on the other side.
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I should also mention that I don't think it's fair to hang this on the ebay vendor - i bought two sets of adapters (4 total) because I wasn't sure how many I'd need, and they're all identical. I think this may be a BMW thing. Or i'm just lucky.
Anyway, back to installing speakers. The stock speaker has three mounting points, and the JL has four (two that you snap off earlier to fit). I could have left it hanging on the two points, but that seemed likely to cause a lot of vibration and noise not to mention crappy sealing. I thought about cutting up some scrap metal to catch the 3rd mounting point when I realized the Jeezus clips shipped with the JL speakers might offer a solution - they had a hooked tang on one side, and the offset of the screw hole was just the right distance. I just had to drill out the snail shell screw hole to clear the stock BMW mounting screws. A 3/16" drill bit worked great. Use a vise or maybe visegrips if you've got nothing better - the clips are spring steel and will grab, which will seriously fubar your fingers if you're silly enough to try to hold them by hand while drilling.
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Here's a shot of the third clip, installed.
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Much happiness. Cervesas, all around!