If you determine that the transducer is not the cause of your interference, you can try placing a ferrite choke core on the power lines, positive and negative leads, before you rewire. Radio Shack also has a closed ferrite that you can wind the power leads on. I have used these before and was effective, but every case is different. Ferrite Core
If you get to the point of rewiring,try to wire the Sonar positive to the Battery Switch terminal that the Battery Switch feeds the console and wire the negative to the battery, this way you still have the function of the Battery Switch. If that does not work than wire it directly to the spare battery.