I have not started the installation. My BMW dealer did not have in stock the door sail to install the tweeter ($32 each). I should get them in a day or two. Then if I have the time, I will do the install this weekend.
For the E90 LCI, the signal goes from the HU to the woofer under the seat (pin 2 and 3). The signal is then bridged to the door speaker (pin 1 and 4). I do not know if there is a capacitor in series or not at the woofer location to send the signal to the door speaker. I will know when I will do the install.