I have a 750il stereo system...

[shogun]

There is some info on the 5 series board where I am asking for a owners manual of a Sony audio. I do not know if it is the same, as you do not indicate any type and model. Have a look at that thread.
If it is the same, the I have got the following info.:

It was not fitted to any vehicles destined for any markets outside of
Japan,
due to the fact that the FM frequency range is 76-90. That means Japan,
Japan, and only Japan

So never fitted in Europe, or Singapore, or Hong Kong, or USA, or NZ, or

Aus. (because all these countries have 88-108 like we do)

If it has standard black square BMW connector on the rear then the
cabling
is:
Large multi-pin connector

1 = Loudspeaker, front left, +

2 = Loudspeaker, front right, +

3 = Loudspeaker, rear left, +

4 = Telephone mute

5 = +12V ignition

6 = Loudspeaker, rear right, +

7 = not used

8 = Loudspeaker, front left, -

9 = +12V battery

10 = Speedo signal for speed sensitive volume

11 = Loudspeaker, front right, -

12 = Loudspeaker, rear left, -

13 = Illumination

14 = Loudspeaker, rear right, -

15 = Ground

16 = Automatic antenna or on-glass antenna amplifier or sound Hi-Fi
amplifier

17 = not used


--------------------------
Model Manufacturer: Sony
Model Number: CDX-5B817

To find a manual, you can try
http://www.partsolver.com
Sony Japan does not have one.

One thing is very important. the speed signal cable. If you mishandle that, you will have problems.
Check also with Sony online dowloads
http://esupport.sony.com/

More details from the 5 series board here on my Sony
http://www.bimmer.info/forum/showthread.php?t=9640

[Whit]

Shogun, thanks for all of that. I should have mentioned that the wires and harness in the dash are intact and not cut. No cuts in the cable (DIN style with pins) in the back seat armrest. Intact.
I am actually wondering about the hookups in the back. I am thinking this is just a 4-speaker system powered by the head unit. It has no amplifiers, correct? In the trunk, it looks like an antenna male plug (or similar), with cut speaker wires(but the color codes are there) and an intact cable with pins in trunk. I think if I plug everything in and wire up the speakers, it may work.
Just to be clear, there is no hidden factory amplifier in this car, right?