I don't know for certain, but if I had to guess I'd say you are probably pretty close at 9V. I know Ford says something silly like 600W total for the B&O sound system, but that's
a) Probably Peak Power, not RMS
b) Includes the subwoofer in there
c) The sum of all the speakers added together
So I would guess probably somewhere in the 20-30W per standard channel, and I think around 150W for the subwoofer.
At 30W RMS, you are at ~about~ 10-15V RMS (but that's all RMS, it can peak much higher), but I bet I'm being generous there, and your estimation of around 9V is probably pretty good.