You either have a poor ground from batt to frame/engine, or the starter is
shot

My Escort's starter also shorted out.  I don't know why, but all over
the windings was powder from the brushes.  Unfortunately cleaning it
off with electronic parts wash didn't help, so maybe a winding had
shorted.

Test the starter operation using the starter relay, not a direct
connection.  It's a lot easier and it may help you to pinpoint your
problem.  Use a remote starter switch or some heavy duty test leads.
Otherwise make sure your grounds and starter connections are clean.