Domminus find another audio shop.
Or learn enough to do it yourself.
No reason you can't use the existing wiring unless you are
exceeding the current capacity of the wires.
Most people don't rewire here. They may run another set
of wires for the doors if they want to mount the crossovers
in the trunk.
If you believe in the Monster wire theory from Home stereo
you might want to rewire but its not required.
If you are not running any amps the shorter route would be splicing into the existing wiring , haven't seen anybody do that
though.
http://www.e90post.com/forums/showthread.php?t=138949
There are side effects putting in an aftermarket Head Unit
is part of the reason its not done more I think.
If you have the L7 System you will need to run wires to the trunk for amps maybe(Amp Input signals) . I am not sure if the car is pre wired for analog to the trunk with the L7 stereo.