Audio Systems: Adding a 2nd stereo to a new VW TOUAREG, alpine amplifier, alpine amp

Question
I just bought a new touareg and the sound system is not acceptable.  I do not want to change the look of the dash board and I am afraid to disconnect the factory stereo because it is tied into the diagnostic system of the car.  My Idea is to install a complete separate system that shares the speakers.  My concern and question is regarding sharing speakers.  If i take the existing speaker wires and split them indiviually with one set going to the existing system amplifier, and the other set going to my Alpine Amplifier, will the signal back up into the amplifier that is off at the time?  What would happen if both systems got turned on at the same time? Do I need to create a switch with 8 poles on it to manually switch the speakers?  I know that Diodes allow current to flow in one direction and not the other, would a diode in each speaker wire solve the potential problem or would it destroy the sound quality?  The existing system has 11 speakers, each with a home run to the amplifier and passive equilization.  I would need to tie each of the 3 front (4 ohm treble,4 ohm mid,2 ohm bass) together to connect to my alpine amp, would this be a problem?  Any suggestions would be welcome.

Thank you!

Answer
Hello Didier,
By connecting a second unit to the existing speakers you are opening your self up to all kinds of problems. I'm not familiar with this vehicle or with the factory sound system in it. Have you contacted the dealer about purchasing a premium stereo package for your vehicle? this may be the best way to get a better sound system short of rewiring the speaker system. You may also ask your dealer if an external EQ is available that will plug into the existing radio to upgrade the sound to the speakers.

TOM,
T&D ELECTRONICS
tdeser@zoominternet.net