1979 Ranger 12/24 Trolling motor issues

I was hoping someone could give me some insight on what to do... I have 12/24v trolling motor I believe its a motorguide but not for sure. Anyway it has a four wire system. When I got the boat one of the breakers were blown. I could tell the wires running from the batteries to the panel in front have been replaced with some rather hefty gauge wire. To my problem the trolling motor works only on the 24v setting even when I have only one battery hooked up. I pulled the plug apart and I noticed there was only three wires going to motor and inside plug was a strap that jumped from one terminal to the other. I'm assuming thats how it gets the 24v.What I was hoping was that when they ran those new wires to the front panel they mixed up the wires. Does anyone know how to wire these up that could give me a little insight or have the wiring diagram for it.