Most British cars including the MGB were wired at the factory using specific wiring codes for different purposes. If you’re fortunate your MGB wiring harness won’t be too mangle by a previous owner. This particular wiring code chart is for British motorcycles but many of the colours seem correct for cars as well.
On a 1996 infinite j30
For a 1996 infinity j30 replacement deck for Bluetooth media
Need the bose stock wire color key