Ha, what amplifier?

I have a sneaky suspicion that the basic audio system in the UK had an amp in the head unit, but I might be wrong. That's the problem with BMW electrics, huge variation for different markets and different option levels and it seems no definitive answers to most questions.

I'm rather hoping I can find the location of where the amplifier would be in the rear (or maybe is) and use the speaker wires heading off into the car and plug my amp in there. And if there are pairs to each of the door speakers (i.e. 2 pairs from the amp location to each door, then I can keep the crossovers at the back with the amp.

I've had a look at where the subwoofer would go if it had one (which it doesn't, yet). I hoped there would be a bunch of (twisted) wires ending in the plug that would connect to the sub, but although there are bundles of wires containing twisted pairs (i.e. speakers?) I've not been able to find any trailing bundle with a connector on the end. Do all cars have the sub wiring there and simply not use it if no sub was fitted?

Am I right that all speaker wires are twisted pairs and all twisted pairs in the harness must be speaker wires?