there is a relay ($6) in the distribution box on the drivers side fender in the engine compartment.for $6 proabably not a bad start. you might even be able to switch with another one in the box and i think there is a fuse in the box also. then the starter interlock switch that you mentioned as well as possibly the ignition switch. have you checked to see if you have power at the starter. any other symptoms?
2004 Ext Cab, 4WD, 4.0, Auto, 146k. (4.0 engine is out of a 2009 Ranger w/96k) ( as of 3/10/2016)
2004 Ranger, Ext Cab, 2WD, 3.0, 5 speed , 4WABS, crank windows, no cruise, Edge 116k.(6/5,2014) (SOLD).
1999 Ford Ranger, Reg Cab, 2.5, 5 speed, 2WD, no AC, 150k (SOLD).