Not sure how your install is currently set up, but the RCA jacks are just 4 wires: Left Channel, Right Channel, and 2 Ground (one for each channel). Should be simple enough to take your existing hardwired connector (the one that is connected to the Zumo) and solder a butt connector to each wire (doubling up the 2 grounds to your current single ground) and shrinkwrap the connections (both individually and then a larger one around the entire group). That way you'll have a waterproof cable splice...
Or, if your 660's mount is like my 550's, and you're using the 3.5mm output jack on the mount to connect, you can also just get a new 3.5mm male connector to directly connect to the waterproof connector without the wire splice.