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Was this an aftermarket alternator or the factory one on the truck? What about the replacement one?
When you put your volt meter on the battery when running is the exact volt reading 15.000 volts?
Any aftermarket equipment on the truck?
Any recent repairs before this issue?
Have a volt meter handy?
Does this light stay on solid all the time now or does it come and go?
Junction block is another name for a fuse block, what I am teling you here is that you have voltage at the top of this white wire where the fuse powers it but not at the end of the wire where it connects to the alternator, you either need a new fuse block here of the wire is bad somewhere. The alternator and battery you installed are not the issue here.