I have a BMW with SA676 (HiFi sound system), which connection cable kit do I need for the audio signal tap?

You need for vehicles with original amplifier (equipment SA676, HiFi) the BVAK (BMW amplifier connection cable). Thus, the stereo front and rear signal can be taken directly before the original amplifier. An integrated Hi-/Low signal conversion takes place in the connector. An additional RCA cable is required.