Ok so I just went through this same thing about a month ago with my 315D. Except like an idiot I purposely cut open the little black sealed piece on the wires to find out what was in there destroying the NTC thermister. I refused to believe that this part I destroyed was some magical piece that only Minn Kota had. So with the other cords I cut leading to the batteries I tested the resistance at 77 degrees Fahrenheit/25 degrees Celsius which I guess is the temp that thermisters are calibrated at and found the closest NTC thermister on Mouser which was 250K NTC thermister
https://www.mouser.com/ProductDetail/803-254JG1J. I soldered this between the Green wire and Red wire ( the white wire coming out of the splitter is the red wire ) and used adhesive heat shrink over the solder connection to seal it and installed new fuse holders and new battery terminal connectors and fired it up and it worked. Sorry I made a long story out of that but I didnt want you to think I Just made up a number and that you should test the resistance between your existing green and red wires on the working cables to make sure yours is the same as mine was. but as cheap as thermisters are you could just give it a shot.