There are only a handful of primary manufacturers in the “12V” accessory market (as they call it.) There are also a large number of one-off products designed and manufactured to solve a specific vehicle and model need, such as “AUX in adapters for late ‘90s Jettas” or the like.
In addition, in these niche markets, sometimes one manufacturer makes the adapter and a number of other companies buy it and put their sticker on it, but don’t actually make it. Some companies will also clone other companies’ adapters, selling knock-offs, so the standard make-sure-you-can-return-it-if-anything-goes-wrong consumer advice applies.
Here’s a list of audio adapter companies:* http://www.aampofamerica.com/ * http://www.pac-audio.com/ * http://www.blitzsafe.com/ * http://www.soundgate.com/ * http://www.pie.net/