Take a piece of wire. Put one end on the battery positive. Back probe the LG/RD hence creating a "jumper" to power the LG/RD from an alternate power source.
Anytime power is injected from an alternate source it's best to use a fused jumper.
Powering the alternator LG/RD wire from an alternate power source will "prove" that the alternator itself will work IF provided what it needs.
I figured that was probably it. I used a 20amp fuse and jumped the batt + to the alternator LG/RD and made no difference. I saw another post you made from a few years ago where you mentioned the possibility of the alternator being incorrect and suggested jumping the #1 and #2 pins on the alternator connector, which I tried also, to no avail.
Also tested resistance between the alt post to the battery, and the case and got OL with the positive probe on the post, and 0.00 with the pos probe on the case. This was using the lowest ohm setting. From my understanding 0.00 indicates a short.
On the cluster, I was able to verify continuity from the #5 LG/RD pin throughout it's entire circuit all the way across the board, until it reaches a diode just before the larger harness on the other side. I get OL both ways on this diode, but do as well on every other diode on the board except for 2 or 3 of them, so I'm assuming they just have some sort of protective coating on the ends or something, thats preventing a good connection. I have no continuity through this diode (nor any similar diodes on the board) from any points before or after though. #5 still has continuity to the #12 pin in the other connector though. What circuit is this pin for? Having a hard time finding a diagram of these connectors.
Now, strange turn, you'll probably jump down my throat over this, as I know it's something you're not supposed to do. I'm not sure why I decided to try it, but I did. I quickly touched the same 20amp fused wire to jump the + batt terminal to the alternator case, and the voltage immediately jumped up to ~13.5 and remained there until the car was shut off, then it had the same no-charge problem when started back up again. This completely baffled me. Now I'm not sure if it's the reman alternator, the cluster, or both.
EDIT: I got this alternator from somewhere else as a reman for an '01 Mustang DOHC', but at O'Riellys, the part number on the case (Ultima 01-3729) rang up as an alternator for a Crown Vic, forget the year they said. I brushed it off as incorrect since those all had the 2v engine which afaik aren't interchangeable, and I was unable to find anything online about that part number aside from a link to O'Riellys of a completely different unit for GM applications. However after doing some more research, it seems some of the Crown Vics could have used a similar alternator, and the wiring on the harness in the 03+ years is different. The LG/RD wire on the 03+ Vics goes to the PCM instead of the charge light, which would definitely explain some of the issues. Is this actually a possibility, and how could I test this?