what would stop the alternator from charging on my 1999 ford taurus

Check for blown alternator fuse or fusable link in wire going to alternator.

Did you check the battery first load test it? what was the output of the old alternator before you changed it? did you check the alternator with a volt ohm meter? or did a discount auto store bench test it. as mentioned already fuse, fusible link and double check grounds.

Here's your problem! Look at the main fuse box under the hood. There's a 175 amp fuse there that has to be blown! Good luck.

Yes did all that the battery light came back on and my radio won't come on