1999 ford mustang 3.8liter-engine getting too hot

System diagram and testing would need to be done , to find the source.

I believe your electric cooling fan is bad - I think if you jump the switch contacts on the switch it should go on - the main thermostat for the engine does not turn on the electric cooling fan. I almost blew my engine trying to limp home with a bad electric cooling fan, as it caused the upper rad hose to burst - which I bandaged - then it burst again right next to the bandage within a mile or so -