About a month ago my battery light came on and the alternator stopped charging my battery. I went underneath and rewired the alternator, ensuring the connections between the wires were solid. The battery light went away, and the alternator started charging my battery again. About two weeks later it came back on. I checked the wires, and it didn't fix it. So, I replaced the alternator. The problem still persists. I'm ordering a new pigtail plug to replace the one that currently goes into the voltage regulator. The wires I initially checked go to the rectifier I believe, and they do not appear to be the problem now.

I had my battery checked and load tested and was told that the battery is fine. It charges when I have it hooked up to a battery charger, but when I start and run the truck it reads around 12.5 and decreases every time I shut off and start the truck. The alternator is new, so my thought is that it's a wiring issue between the alternator and the battery.

I'm just not sure where my problem is, and I really need any helpful input. Does anyone have any idea where my problem might be and how I can fix it? If I've left out any important details or further descriptions are needed, please let me know, I'm still rather new to this, thanks!
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1987 F150 XLT 4x4 5.8 L