Does anyone know where I can find tear downs or schematics or some sort of information on the electronics inside the box within those aux input adapter kits? Example:

All I need is a way to plug in an aux cable to my 2007 Honda Accord so I can play audio from any device with a 3.5mm headphone jack.

I don't need to control any device through my car's buttons or even charge the device through one connector. I use a variety of devices, so getting a kit for a specific device (i.e.: an iPod) is pointless. Furthermore, the reliability of a lot of these boxes worries me, so if it's possible to DIY more of this, then that would be best in my case.

Any suggestions are greatly appreciated. Thanks.