All of the wiring in BMWs is excellent quality twisted pair copper. The wires from the amp to the speakers vary in gauge dependent upon where they terminate. The mids and tweets in the door and rear deck run on 18 AWG wire, and the woofers in the kickpanels and rear deck run on 16 AWG wire.
Unless you're running in excess of 100W RMS constantly on all channels (not likely) the stock wiring is more than sufficient for most systems.
Two things you should be aware of, in case you didn't already know. 1) BMWs that don't use fibre optic connections from the head unit to the amp, use balanced differential outputs (again through twisted pairs). 2) The reason for the BDOs and twisted pairs is because most BMWs are very noisey electrically- probably because the battery is in the trunk, and they have fairly complex electrical systems.
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