if i can do it #1 how ? #2 what is the drawback to do it this way ?

 

You need a device called a LOC (line output convertor). It for the lack of better words, splices into one pair of speaker wires, say the front left and right and samples the signal going to those speakers. It then derives a signal that is changed to a rca type lead. Do not buy a cheap welfare LOC, buy a good one. Scosche, Pac and others make some great devices. For a even better unit consider a Audio Control LC6 + bass remote.

www.scosche.com
www.audiocontrol.com

 

There are adapters that you can get for some car that will allow your stock headunit to have RCA outputs. That way you wont need the LOC. But I have only seen some Euro car with this option. You dont drive a VW do you???