I'm personally a fan of Mediabridge cables because they're well shielded and very sturdy for pretty cheap. As above, shortest one will be best. There is nothing wrong with leaving it plugged in 24/7. Also, for best results, turn the volume on whatever you have plugged into the AUX port up to max before turning the volume on the car stereo up.
Edit: FWIW - since this is an ANALOG cable, quality/shielding does matter some. For digital signals, like HDMI, it either works or doesn't so going cheap as long as it works at all is fine.
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'13 128i | 6MT | SGM | Black Boston | Black Gloss | Lighting | Premium | Tech | HK | Anti-Theft | [CDV-Delete | Clutch Stop | 30% Pinnacle]
Ordered 2.23 | Production Begin 3.7 | Ends 3.15 | Released to carrier 3.18 | At port 3.19 | Departed 3.23 (K-line to Brunswick) | Arrived 4.7 | Truck 4.12 | Dealer 4.19
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