I have never heard of anything specifically referred to as a "Bose adapter" (though I'd imagine whatever that might be would be expensive and overrated, just like anything else Bose...), however, if your plan was to upgrade your stereo with an aftermarket amp and speakers, what you were most likely being told to purchase would be something to counteract the extreme equalization coloring that Bose equipment performs on the audio that passes through it. A quick search of this board, or any other board where Bose systems (or any other system that performs equalization at static levels), will shine a little light on the subject of factory headunit equalization.

In a nutshell, car audio manufacturers who provide factory systems typically, especially in newer models, perform audio equalization (I'll assume you're familiar with that term) to compensate for shortcomings in the speakers, due to size, cost, or design restrictions. The problem comes along when you connect higher quality speakers to this audio, because they are now pumping out "colored" sound that was designed for inferior speakers.

What these equalization boxes do is perform basically the opposite eqalization to what is going on in the headunit. Some allow you to adjust the frequencies and sensitivity manually, and some are fully automatic, like the soon-to-be-released JL Audio Cleansweep.