The perfect stereo.
For a long time I struggled with my car stereo and its inability to play right from my phone. I snipped the cord from the tape deck and installed my own 3.5mm jack on the end which worked for a while, but due to the gain mismatch and the stock 1995 amplification, it just didn't work out. The previous owner had replaced the speakers with some beautiful Kicker i90s, but the stock stereo couldn't keep up.
I saw the picture on the left on my Facebook feed and realized that although it was a joke, it carried a lot of truth that I wanted to make happen. It would be way cheaper to funnel all of the money into the backbone of the system, the amplifier, rather than paying money for a "good" interface. (go look at car stereos on Amazon- what about this specific thing makes it immune to all of the rules of desgin?!)
I designed the part in about 15 minutes using IronCad software and the ISO standard for double-DIN stereo mounts. I added the blue part for decoration (another 15 minutes) and couldn't resist slipping in an analog clock to fill some of the space. The green indicator shows power and the (upside-down, whoops) power switch cuts the main power to the amplifier.
Behind it It simply a cheap 4-channel amp with RCA inputs. I can plug an aux cord in and after a little tuning, the little amp kicks out more sound than the stock stereo ever did.
Total cost: About $0.50 for the plastic and $40 for the Amp. I had the switch and indicator from a previous project although they wouldn't add more than a couple bucks.