This was my answer to a problem I had connecting a Garmin 176C and a Nasa DSC radio, it may help you :-
I have spoken to Garmin and Nasa and have the answer. The problem I had was that when my broken radio was taken out I was left with a brown and a white lead from the Garmin, the radio had a brown, black and a red lead coming out, I thought I just needed to connect brown to brown and the red to the white, as the black was going to the negative power elswhere, but NO as that did not work.
I must ignore the white from the GPS as that is NMEA in and not required.
The Brown NMEA out from the GPS should connect to the Brown NMEA lead from the radio
The Red lead from the radio must go to a negative supply, this supply could come from the black lead that is powering the radio.
You should NOT connect the red lead and black lead from the GPS straight to a negative source as it will cause a problem. These two leads should be connected together and then a lead comming from both goes to the Negative power source.
Clear as mud maybe, but I will give it a go on Saturday and see what happens.
By the way it worked
I accept no responsibilty for anything going wrong, you should seek independant advice before following these instructions