So,
S is start Yellow/Red
C is choke Purple/White
A is accessory Purple
M is ground Black
B is battery + Red
M - threaded hole - STOP shuts off motor - Black/Yellow
I'd start by disconnecting all wires from the B terminal except for the original RED wire. I'd also trace that wire and make sure nothing is tied into it. That's a direct wire from the battery. It has 12 volts at all times. If there are several wires on that screw, remove them all and only connect the one that has 12v. I'd disconnect all wires from the B terminal before plugging that big 1" diameter plug back together, and find the one that has 12v and connect that to the B terminal by itself. Next step would be to check the M ground termina and make sure that the wire is in good shape and the screw is tight.
Lots of boaters, as they add accessories to their boat, they attach the power and ground wires directly to the ignition switch. If/when those accessory items go bad or they aren't wired correctly, they will cause that fuse to blow. I hook gauges, lights, stereo and other very low draw accessories to the A terminal and have inline fuses for lights and stereo. Bilge pumps, livewell pumps and other accessories with larger amperage draws, and depth/fish finders, I hook directly to the battery with inline fuses and switches.
Good luck with tracing down the problem. With the age of the motor and control box, the colors on those wires might be hard to figure out. Sometimes, I very carefully cut away a little bit of the insulation on that 1" diameter cable that goes from the control box to the motor where the wires aren't as faded. But be sure to wrap that up really good with electrical tape to keep water out.