Current wiring harness has a two wire plug for a sensing circuit but the new alternator does not have a provision for connecting the plug. One of the wires goes to the ignition switch and the other to the alternator lamp on the dash.
Check voltage between both wires with the original alternator and the engine running as well as stopped (but key on).
my guess is the original alternator closes the circuit between the positive (coming from ignition key) and lamp.
As for the replacement alternator my guess is it supplies ground from its only connector when the lamp should be lit (please measure it with voltmeter!). If my theory is correct you should wire a relay in the middle to allow the lamp to be lit, since you need a positive for the lamp.
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