Im not sure if its compatible with this model, but I believe you would use a NMEA Network to link electronics. (Like "SeaTalk" with Raymarine) But are you sure you can link a radio to the m68?
I took a look at it and I think its compatible with NMEA, you would have to check with Lowrance for indepth instructions on how to do it though (Im a Raymarine Guy ) What boat is it on?