The perfect stereo.
I hate car stereos. For a while the best I could do was soldering my own 3.5mm jack to the main board of whatever crappy stereo I had at the time, but the gain mismatch made the sound quality terrible.
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 is it about this specific thing makes it immune to all of the rules of human-centered design?)
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.