Your 'diagram' of the 12 pin connector matches the pic in the book.
Looks like only pins 9 + 10 are used (comms bus) and even then only if satnav is present.
So, based on that, the 18 pin will be as per your second option - ie. 1-18 ...
1=B+, 3=Amp+, 5=SigGnd, 7=mute, 8=FL, 9=FR, 10=Acc, 12=Ant+, 14=Gnd, 16=beep, 17=RL, 18=RR
Hope that helps