I know it can be done, but is it a good idea and, if so, what's the best way to go about it? I have a sub and amp from a previous vehicle and would like to install this in my 4th-gen Max to take the bass load off the existing speakers, but don't won't to spend any money on a head unit or other speakers... the existing Bose is good enough for the time being.

 

Search the forum for CobyMoby (I think) he sells PAC adapters which will give you RCA level outs from the stock Bose unit.