What size fuse is it blowing? Any chance you could measure the current?
Fuses blow in 2 ways. One is a short which is obvious. Cable fault as above or alternator winding fault. The other is an overload. So as an example if it's a 10 amp fuse it will supply 10 amps forever. Go to 10.5 amps it will blow after a couple of hours. And the time gets shorter as current increases.
Could mean the difference between a shorted cable and a bit of water causing a high resistance earth fault.
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