Just for "because" what is the car that it is installed into? What year/ model?
The older style, that is, 69/ earlier is a simple circuit. The ignition "run" wire branches off to feed the VR which MUST be grounded. The field wire of the VR (green) goes to the alternator.
The alternator output (the big wire, stud, BLACK) goes through the bulkhead to the ammeter circuit, back out the bulkhead on big RED, through the fuse link, and to the starter relay stud (junction point) and to the battery
So for starters get the battery recharged, run the engine on fast idle to simulate low to medium cruise speed/ RPM
Measure battery voltage and then voltage at the output stud of the alternator. They will both be likely low, below 13.5V
If the alternator is very high, say, 15 or more, and the battery is low, this means there is a break or bad connection in the output wire
If the alternator is low, below 13, then it is not charging
Now "rig" a jumper wire. Remove the green wire from the alternator field, and jumper from the alternator terminal to the battery. With the engine running, it should bog down a bit, you should hear the alterator whine, and the output voltage should go up.
If not, recheck the voltage at the output stud. If the voltage is still low, below 13 with your jumper connected the alternator is not charging.
First thing to do is check the brushes for wear, stuck, grease, dirt, etc. If they seem OK you likely should just replace the alternator.
I would NOT buy another early one unless you are looking to be "restore correct." Get a 73/ later and make sure it has the squarish heat sink. "Square back" rather than "round back" like you have now. All you need to do with the newer one is to ground one of the 2 field connectors, then connect as your old one.
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Now, in the above tests, if your jumper wire DID CAUSE the alternator to charge, you now have a problem in the field/ VR circuit. Either the green wire is broken, the VR is ungrounded, the VR is bad, or the VR is not getting ignition "run" power to the VR
Checking for power is easy, simply use a multimeter at the IGN terminal of the VR. Make absolutely certain the VR is grounded. The green wire only goes one place---from the VR FLD connection to the alternator field. Easy to check.
It is certainly possible the new VR is defective (out of the box), or that the alternator field is shorted, and blew up the new VR.