I'm probably the last person who should be replying to you, so take it for what it's worth.
I checked the owner's manual and you do not have a 50 amp fuse. Did you mean 40 amp? All the fuses are under the seat, on the driver's side, but I think you know this and can't find the right one. The only "fuse" for the radiator system is a breaker that can be reset if the fan is not working.
See attached page from the manual. The pic on the left of the page is showing the 40 amp fuse for the power steering.
Also, The manual says, "When you touch the cooling fan,be sure to disconnect the (–) cable of the battery, since the cooling fan can turn automatically even with the ignition switch off."
So, there IS supposed to be a connection on the battery negative.
You sure the previous owner cut the red wire??
Also, just want to be sure you know that the fan does not come on automatically when the engine is started. It only comes on when the thermostat reaches a certain point and you won't even know it's on until you stop and turn off the engine. The fan will continue to run for a few minutes to cool down, then shut off. Well, this is how mine works and think it should be the same on yours.
Hope something I said is helpful to you. Wish I had more to offer. That's a great mule!