Got the Primera home, and decided (after washing it) to have a look at the radio wiring. Previously, the switched live had given up the ghost, and was bypassed, so it now only worked with the headlamps switched on.
After pulling everything out, I reckoned on swapping it for another (better) Blaupunkt head unit I had knocking around the place. All going well until I noticed the aerial leads were different.....
Went back to putting the original unit back in - but couldn't work out how the wiring had been bodged in the past.
4 hours later, I've snipped everything back and am currently mating a Sony ISO loom to the original Primera wiring. In the dark. And now with a flat battery due to me working with the interior lights on.And I _still_ can't listen to the radio - it's that bloody aerial socket issue again. Still, that can be fixed easily tomorrow.