I don't mean to thread hijack and a little off topic but how hard is it to replace the stock Harman Kardon non-dsp+nav amp with a generic 4-channel amp re-using all the existing stock (2 ohm) wiring? The front driver channels seems to cut in and out sometimes.

I already have a sub woofer amp tapped off the rear leads.

Fix it well....
 

Couple of things.
1. Where you have the dsp setup your best bet is to tap the wires on the speaker side of the amp. Because BMW puts their battery's in the trunk of the car and runs power wires to the engine compartment it turns the entire car into an electromagnet, this will cause noise in your stereo). To combat this problem the stereo's in these cars use what is called a floating ground or balanced rca setup (higher voltage and impedence to combat the interference). Because of this the dsp setup has no line level rca's from the head unit. All splitting of the signal's is done at the amp, also all sound processing is done at the amp. therefore if you want to be able to control your new sub by adjusting the bass on the head unit you want to tap a wire on the outbound side of the amp (i.e. one of the rear woofer speaker leads).
2. If your amp has a lo/hi converter in it you should be all set. Usually any wire with a brown stripe is a negative and most wires are twisted pair to the speakers, this should simplify your wire tapping. I would still recommend a dedicated hi/lo converter because the one in your amp probably isn't designed to accecpt a high power signal that your amp will be putting out. you can pick up one at walmart for around $10
3. Most of these cars have a grounding problem with the rear lights (the design was faulty and caused all sorts of noise problems with respect to aftermarket stereo equp because of poor grounds). When you install your amp I would recommend grounding it to a specific spot......In particular the fabric strapped tire removal tool in the car is connected directly to the frame by 2 allen screws on either side of the jack, at the front of the rear compartment. I have had good luck grounding off of these locations as it seems to be isolated from the rear portion of the frame. Also, you won't have to drill any holes in the car, as a standard "large" terminal ring usually works with this bolt.
4. If the problem still persists I would recommend checking the routing of your wires, or replacemnt of the wires. As previously stated these cars have all types of ground noise caused by the rear battery placement. Sometimes re-routing the cables can solve many noise problems.
good luck

Wow thats good info I am going to relocate my amp grounds to this locale.

one more thing
 

sent a long reply to your email last night, forgot to include that the factory amp divides the signal between the front door woofers and small dash speaker; most likely you'll combine them on your front channels and you will need to use a passive crossover to keep low frequency out of the dash speakers, or they will blow. it can be as simple as a 100-400Hz non-polar capacitor inline on the positive lead, couple bucks from radio shack.

i will certainly test moving the ground as you suggest! unfortunately i'm not home till weekend after to start playing with my car again :(

waw! i didn't know about balanced RCA! reading through i kind of feel - that might be why my cars factory amp doesn't like my own amps RCA outer (which is most probably grounded, right?). because it basically grounds a signal that should not be grounded!

if that's the case would a ground loop isolator fix this problem?

that brings up one question though. how come other ppl are able to hook up an amp easily into onto the factory amp output twisted pair, if that's balanced RCA? cos the externally fitted/3rd party amp is most probably not a balanced RCA setup.

faranks, 'long reply'? didn't get that (unless its the one further up this thread).

no, that was to the other poster, 350z

These guys create upgraded sound systems for BMWs and have great install pdf's on their site that many other people use to install their own gear. The pdfs on their site might help you out (ECS Tuning is in no way connected to BSW just sharing info).

ECS Tuning