Use generic solenoids and wire it yourself. Its just a simple reversing DC motor. In fact, pull the existing solenoids apart and clean the contacts. Where is the power source and what size cables do you have? Dollars to doughnuts you have a bad ground. Check the cables at both ends, battery and motor. Does the solenoid click when you move the switch?
Actually, cut it off and throw it away, you already have a good engine and a good rudder. I can see the purpose of a bow thruster but the stern thruster is pointless. Your rudder steers the stern of the boat, its controlled by that big round thing, called the "helm" or the "wheel" in the "wheelhouse." Its the bow that gives us the grief because it isn't connected to the wheel. Sorry (running and ducking for cover).
Don't believe everything that you think.