attached a 3.5mm stereo plug and insert into the AUX input jack on the front of the radio.
I did this with my Garmin Street Pilot on another RG. It worked, but took some electrical engineering to make it happen. There was too much gain coming from the Garmin, and it caused the sound to be distorted after about 30 minutes (with potential to damage the radio). Took it to an electronics genius friend who coached me into soldering a component (I forget what) inline on the output wiring to fix the problem.
We also used a small plug on the Garmin side, into the body of the Garmin. This plug disconnected the Garmin internal speakers when I plugged the cable into it. The cable was male/male. It was a real clean set-up when done, but a royal pita to construct. Hope this helps!
[PS - went with RoadTech Zumo this time]