Also check the starter relay on the driver's side fender, and the solenoid on the starter itself. The mini starters that Ford started using in the early 90's have a solenoid that both engages the starter gear, and open/closes the high current supply to the starter. The relay on the fender simply switches the solenoid on and off.

I'm not exactly sure of what a starter interrupt relay is. If I had to venture a guess, I'd say it has something to do with an anti-theft system or perhaps is part of the neutral start or clutch on off switch.

Bart