UFO-LOW has a good point that the battery should be in good condition and fully charged to be able to evaluate the charging system.
Every thing JFB said is spot on and the schematics are good to see the circuit.
The connections on the factory amp meter are known to get over heated and burnt . A peak at them is worth while. Remember to take the battery cable off when in the sub dash. It is "hot" and not fused!
Until you examine and repair all the connections in the charging circuit , you might be wasting money to just throw parts at the situation.