FWIW, twisted pairs only really work well in balanced audio - not the un-balanced audio found in car audio systems.

The unshielded audio wires could well be a significant source of noise (especially in a car full of high current PWM signals!) but you still need to sort the grounds and power weirdness.

The best place to connect the power is where the OEM radio was connected - assuming it was in the dash (MID + Business/Professional CD etc.) - I don't know what your car had originally.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)