Hi,
Red wire, from plug on (new)radio ( acc ignition switched ), this is the wire to connect to the switched live on the original car plug, regardless of what the instructions seem to say
Yellow wire, B+ constant power supply, from radio plug not from the socket end) goes to your new wire from the battery via a new fuse 10a not 5 as i previously said as this is the main amp power feed, of its own.
It's very simple as long as you follow the wiring diagram not the instructions-I know this sounds wrong
I've lost my picture of the wiring for the original radio plug, found it on my Google cloud