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