It shouldn't matter. You have a red and white RCA cable with all 4 wires in one cable. I just happen to have them separate. On your cable, you will run the red wire to the + of one speaker (usually right speaker), and the white to the + of the other speaker (usually left speaker). However, which one you choose doesn't really matter technically. If you find that when you adjust your balance to the right and it goes left, then you can just swap them at the solder point, or at the connector.
Now, for the shielding or (-)part of the cable. On most vehicles, they are tied together. I think this is the case with the BMW...or at least it was with mine. So, I believe you have all your connections right. I would check the points where you plug them into the radio again.
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