Sounds like you have a dead battery.
If your 1999 Ford Mustang cranks but won't turnover you may have a problem with the starter, the solenoid, or the alternator.
Does it turn over? Does it have fuel, spark, compression?
Cranks is the motor turning over, But cranks and won't start means either you no fuel or no spark or both.
if the engine cranks.... then it is not the battery or alternator.... look at the primary electrical system and fuel injection
are you getting spark?
bad fuel pump
An engine which cranks but will not start is not receiving enough fuel or has no spark. A clogged fuel filter or bad spark plugs often cause this problem.
engine cranks over but will not start.
check the crankshaft sensor
If it just cranks and cranks and cranks then it's likely fuel pump. If it died and it won't turn over anymore, likely alternator.
No. It should start. It may run poorly and lack performance but it will start. If you have a no-start condition, (starter cranks over engine but engine doesn't start) check for spark, fuel and compression. You are missing one of these.
For a 2004 Ford Mustang : It was the 40th anniversary of the Ford Mustang , and I believe they called the Over Head Valve , V6 engine a 3.9 liter instead of the usual 3.8 liter