Most NMEA signals consist of "DATA" and "SHIELD" or screen to/from the GPS unit. I've not worked on the Cobra F75, but the data input at the back of the unit may also double as the data to clone or upgrade the software for the radio (this is normally done with the "ring" of a stereo jack.
I would guess that the tip is DATA in from your GPS unit and the sheild is connected to the outer ring - It shouldn't matter if you have a stereo or mono jack, as long as you use Tip and Shield for the GPS data.
Don't forget NMEA sends data at different rates and is configured in the GPS menu so it can "speak" to the other unit ie. PC, DSC Radio, AIS, etc
Hope this is of some help.