I got a 2004 Z4 a few days ago and my first DIY project was aux input. Went on Amazon and got a cable that seems to work great so far and cost only $11. Required a little more effort than the Mini Cooper OEM one, but also cost less. Definitely cheaper than the ashtray kit.
https://www.amazon.com/gp/aw/d/B00PL...?ie=UTF8&psc=1
I bought a coiled aux cable for $6 as well.
