Not sure how your Ranger is configured but my Ford truck the starter only has one input. When it is +12 volts, it cranks the engine. The + 12 volts comes from the starter relay, which is bolted onto the fender as I recall. The starter relay is only supposed to energize the starter when the “start” terminal is +12 volts, which only occurs w/the key in “start”. [ Maybe it is labeled “ign” rather than “start”] I think what I’d do in this situation is remove the wire at the starter relay that goes to the starter motor. Then use a test light or DVM to determine if that terminals voltage is when the key is in “off” , “on”, and “start”. It should be zero volts in the first tow, and 12 volts in the last. The starter motor won’t crank in any of them b/c you disconnected the wire for the test.
BTW, I’ve had trouble with the starter relay before. One time it was just plain broken and needed replacement. Another time it had come loose from its moorings. It turns out the mounting fasteners also provide its ground. If it isn’t mounted soundly, it won’t work.
By contrast, the starter motor on my Corolla has two inputs. One directly from the battery, which is always 12 volts. The other comes from the ignition switch, and is only 12 volts when the key is in the “start” position.