I can't say for sure (because I don't have a Becker manual in front of me) but I'm fairly sure that the telephone audio (ie, what you would expect to hear when on a call) will not be output via the pre-outs - I think it may be the case that telephone audio is only output via the front high level (speaker) outputs, and that if you have your front speakers driven by an amplifier then you're not going to get telephone audio.
If you have music coming out of your speakers then your amplifiers are wired correctly - they either work or they don't. It's not possible to wire them incorrectly (with that loom) to give you music but not telephone audio.
Have a look through your manual. It may be that someone in the menu system there is a setting that allows bluetooth (telephone) audio to be played through the pre-out signal (and therefore through your amplified speakers).
On most car kits that are connected into the audio system (and not played through an external speaker) they work using the high level, unamplified outputs to the front speakers only, which are not it use in your car.
If this is all correct, and your are using non-amplified rear speakers, I can advise a way to re-jig your outputs to get this to work.
