Ford’s usually have the starter solenoid on the firewall unlike lowly GMs which use the solenoid to make the electrical connection as well as shoot the Bendix out to the Flywheel teeth.
I don’t know how a 98 does it but usually, there’s a big cable that goes to the solenoid from the battery positive, this is for the starter. There’s another smaller cable on the positive battery terminal or on the battery side of the solenoid that powers the rest of the vehicle and also gets the charge voltage from the alternator. You could try to disconnect the positive from the alternator; maybe it has a shorted diode or something?
As someone else mentioned, you can also pull fuses until the draw goes away. Before EFI, you would start the engine and then pull the negative from the battery, if it died, the alternator was toasted, nowadays, that has the possibility of taking out the EFI which is no fun.
The radio has a parasitic draw to keep all of your pre-sets. How old is the battery? You would be amazed at what a bad battery presents as.
Let us know what you find?