Here is a link to my photobucket 1971 521 wiring diagrams (2 images). radio wire appears in the #2 image. These files are over 5 meg each is why I can not post a good image of them here. Please feel free to download these pictures to your computer if you like.http://s477.photobucket.com/albums/rr13 ... 69_Datsun/
The radio wire is blue with white stripe according to diagram. On the diagram it is located between the clock & windshield wiring connections.
Hope this helps.