+1 on above. However my vote is not a short but a weak connection somewhere in the electrical path.
Here's some more detailed information on how to perform advanced trouble shooting on the charging system. Howto perform charging system voltage drop test https://www.stangnet.com/mustang-for...-drop-test.56/
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.