i heard i can do a line to level converter and use that as your pre-outs for the amp on the sub and tap into a speaker wire or if you have the Bose use the wires for the amp for the line converter.
Replying late, but I hope this helps. I have installed aftermarket head units in almost every car I've owned. The answer would really depend on your price range. I would definately add Clarion and Eclipse to the Kenwood and Alpine mentioned above, assuming you're looking for a mid-to-high end deck. I have always been very pleased with Clarion. Most of the major brands have everything from a low end to premium lines, but the sound quality and clarity does vary from one brand to another.
It will not drain your battery because its not getting juice from the speaker wire...you use electical wire from ;your amp to batt and if you have a amp 1000 watts or over then get a capacitor...so just get the hi/lo level converter, tap into your rear speaker and then you can keep your stock head unit
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hook up sub for a6? - 
