My girlfriend has an E46 and you readers may find this interesting.
Those not happy with either the standard or HK sound system should be very
careful about installation of other amps etc.
we wanted to install one 12" sub woofer and 1 V12 5 channel alpine amp. we
discovered the following problems....
The BMW system has no RCA outputs for adding an additional amp, and the deck
can't realistically be replaced as it is a custom fit and no standard 1 din
system will fit into the dash.
with out RCA outputs, the only way to add an amp is to clip one of the
speaker wires and add an RCA converter to go to the amp. The major problem
is that the BMW system is closed and actually looks after amplification for
each speaker individually. Once the system is opened in any way a massive
amount of Alternator wine or signal noise gets into the system.
We tried various way of eliminating this but is impossible, no question,
once a speaker wire is opened to add an RCA converter, you will get signal
noise, there is no two ways about it.
To minimize it however, we found the following solution.
Clip the wire at back right hand speaker add the RCA connector there and not
at the head unit as would be the normal practice.
Mount the amp in the trunk fixed to the back seat.
Run the shortest amount of cable possible from where the wire has been
clipped to the inputs on the amp.
Because the battery in the E46 in mounted in the boot, run the shortest
amount of cable (fused) from the battery to the amp, making sure all power
lines are well and truly separate from any signal lines.
Use a noise filter, this made a big difference.
Use gold connections and the best possible speaker wire, power cable and RCA
This setup was without the most successful due to the short distance
between power source, signal and the amp.
There is still noise evident, it cannot be fully eliminated without changing
the head unit to one with RCA outputs, but they don't fit easily.
that's my .02c worth