SilverWidow, I'd like to inform you that the Bose stereo on your Mazdaspeed6 puts out 2v line-level outputs. That means you don't need a Line-Out-Converter. All you have to do is place RCA ends on the outputs from the factory head unit. There is info about this on the "Mazda 6 Club . com" website (can't link directly because of stupid cross-linking rules).
Also, if you are only powering a subwoofer (mono) then you can probably get by with just using the input signal wires to the factory bose subwoofer amp (the sub is powered by a separate amp located on the back of the sub).
Finally, since the Bose head unit puts out line-level quality outputs, there is really no need for an aftermarket deck unless you want a flashy display or exotic features like bluetooth or DVD playback. MP3 playback is a breeze with one of the aux-input adapters or ipod-adapters available. You can also get the phatbox too.