Did similar recently with a Lowrance GPS plotter and my new Standard Horizon radio. Made the connection in/out, out/in using some 4 conductor wire from the pile in the basement. Worked fine for a while - then the GPS would freeze up. Reboot the plotter. Had to disconnect the output of the radio from the GPS to keep the plotter running. I think my problem was the cable - I just used some old 4 conductor phone cable and I think I was getting crosstalk - GPS output coming back in the input. I think I should have used twisted pair cable. It's not like I don't have enough Cat5 twisted pair around the house.. Oh well, I still have to replace the cable - but I'm not really sure what the plotter is going to do with the signal from the radio???