In this recent thread
the poster is connecting an AIS unit with an rs 232 output to a raymarine
plotter (though an older one). It works, despite the fact that the physical connection (ie rs232 rather than 422) is not conformant to the later NMEA-0183 standards. The same should go for yours.
Your mission, should you choose to accept it, is to connect the transmit data pin ("TXD", pin 3) on your rs232 connector to "NMEA in +" connection on the plotter and the ground pin ("GND", pin 5) on the rs232 connector to the "NMEA in -" connection which is the pair of the "+" connection you used. Use the input only pair (orange/white and orange/green) if there's no pressing need for bi-directional communication.
You can buy a 9 pin d shell from radio
shack and make up a cable, or as smac999 suggested, chop the end off an existing serial
cable. If doing the former, use your favourite search engine
to find a diagam for "DB9 pinout", but beware that it's easy to get confused between male and female, front and rear views of the connectors. pin 3 is easy to spot though and you've got a 50/50 chance of getting pin 5 the right way round by chance alone. If chopping the end off a pre-made cable, use a multimeter to find out which pin ends up in which wire end.
Anyone else think that raymarine missed a trick by building all this wireless and network stuff into the new plotters, but then neglecting to think that people might find it convenient to send old-style NMEA data over a network rather serial lines?