Like on most vehicles the smaller wire(s) going to the solenoid are from the ignition, Mike, and when energized they close (unite) the large contacts inside the solenoid that allows high current flow to the starter motor. If the solenoid clicked when the ignition is turned on, but there's no engine cranking, than as Gerald suggests there is likely a corroded or loose large wire that's resisting adequate current flow from the chassis batteries to the starter motor.
If you have a voltmeter, you could see if the large wire into the motor does get juice when the ignition is activated... black meter probe to a clean unpainted metal chassis bolt, red probe to the wire's starter large wire mount. But just because you may see voltage doesn't mean the connection isn't poor and too resistant to enough current flow. Commonly there is no matching large black ground wire because ground is achieved via the starter's mounting bolts alone.
I'm an overly cautious guy, and would at least turn off the chassis battery switch, if not also remove the battery ground cable, before crawling under there to loosen nuts. The large wire means high current that if shorted via a wrench, or already internally, could arc substantially; you don't want that in your face under there when you go to loosen nuts to remove wires for any cleaning and retightening.
Joel
