Often times, you can simply use the old cable to pull the new one. Typically, the connection of the wire to the new transducer is fixed (doesn't come apart). Which means you have to pull the new cable forward. Butt the two ends of cables up to each other end to end (cut the old cable connector off), and tape them up that way. Wrap the tape a good 3' or 4" down each wire for good adhesion. Don't make the tape too thick anywhere.
If your new one already has the connector attached, you can still do the same thing. But do your taping in such a way so that there are no parts of the "tape connection" that can get snagged. In other words, use the tape to make smooth transitions.
The one caveat to all this is whether or not the existing cable is secured along the way, anywhere. If you can't get to those cable clamps/zip ties, then you'll need to use a fish tape.
If you pull a thin, nylon string (1/8"-ish) along with the new cable that is three times longer than the fish run... You then leave a 1/3 at each end of the run and you'll have something for future use.
