Cannot keep it running?

I bought the truck not long ago and I have already put four alternators on it. This last one I noticed the plug was broken. I rewired the plug and but now when I turn on lights or radio or anything it drags it down and will not stay running it acts like it’s a bad alternator, but I’ve had it checked, and every time they tell me the alternator is good. I’ve replaced the alternator fuse it was blown but it still does the same thing.