I just bought an 03 4x4 blazer that used to be used as some sort of company vehicle w radios and antennas. I've checked the grounds, alternator, battery. All checks out to have proper voltage/amperage. The alt plug has 2 terminals. The alternator itself has 3 prongs. When truck is running I can unplug alt and lights stop flickering and the draw on motor elevates a little. I've checked some YouTube videos for some ideas and of course same I'm nonsense that doesn't help. Any clue?

 

my guess wrong alternator

 

You should verify the alternator is the right alternator for your vehicle, like previous comment. You might also verify that the connector only requires the two leads. Some one might have just put something on it that worked enough to limp it to it's resting spot .