My understanding is that the THA275 uses the 12v line from the headunit rather than needing a separate high current feed from the battery. This would just be run in the standard BMW adaptor harness that you can buy for the amp.
The RMS current to each of the channels at it's worse case 2 ohm load would be just over 6 amps so i'd imagine the amp will be fitted with a 15 or 20 amp fuse. The system wiring loom should be able to cope with that (but don't quote me on that!!
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