97 4.3 No Charge

I have a 97 GMC Sierra with a 4.3L in it. The truck has 240,000 miles on it. We just got it back from the mechanic for a bad ignition control module. When I picked up the truck I noticed the battery gauge wasn't moving so I had the mechanic test the voltage and the alternator wasn't charging.

I knew the alternator had a lifetime warranty so I drove the truck home to replace it myself. I removed the alternator and had it tested at Autozone and they told me it tested fine so I took it home and put it back on. Once I put it back on it still isn't charging. I've wiggled the connectors and tested the fusible link and still nothing. I tested the voltage at the battery as well as at the stud on the back of the alternator.

Any ideas what I'm missing?

Also, the alternator has 4 prongs on the connector but the harness only has 1 wire?

Got it fixed! Even after checking every fuse with a multimeter the fuse for the gauges turned out to be the problem. Replaced it and now everything works. The fuse still tests good with a multimeter. Go figure.