If there is an old cable installed, do as himmelrr stated and remove the right rear vent.
Attach the old cable at the console to your fish tape or rods and pull it rearward. Once you get access to the Tape or rods, use the process mentioned above and pull the new cable to the console. I have always wrapped the connecting cable and fishing tape tightly with electric tape. I also put a dab of dishwashing detergent and spread it over the tape. Allows it to slide very easy thru other wires and cables.
My last Transducer replacement was pretty easy. I had torn the transducer off with the help of a stump. This just left the exposed cable. Attached the new transducer cable to the old, and pulled it to the console.
I cant stress this enough. Make SURE the cable is run properly from the mounting location outside the transom (JP routing or however your running it) to the vent area. If you run it incorrectly, the transducer will possibly keep you from rerouting in certain areas, and you will be pulling the cable back from the console to reroute.
Don