I have 1989 Olds Cutlass Supreme and problem is that original radio is broken and I replaced it with a bit older radio, also Din 1,5 but with older plugs (from a Van). I connected all three plugs but there is one more plug with 2 wires - on schematic it's plug with N and N. M is 12V+ constant, ok, but what is N? Schematic shows there is no wire although a wire runs in the radio from that place and there is short wire also on the plug (can't remember the color) so there was a wire when radio were at the van. Radio works, only problem is that backlight of the radio cannot be regulated, it is the same all the time. Also, there is one more plug (1 wire, regular female plug) on the radio so I don't know how to connect these 2 wires.
http://www.corvetteradios.com/gmplugs.jpg
On this schematic upper one, N cable is a mystery and that other cable isn't on the schematic at all. If somebody managed to read this junk and knows how to help I'll be glad.