The solenoid is an electro- magnetic coil, that when energized by the small ignition wire does two things. It pulls a plunger back that engages the starter drive gear(sometimes referred to as a Bendix) to the flywheel. The second thing that happens is that a thick copper disk is pulled back connecting two sets of contacts which engages the the starter motor (the big positive cable that is bolted to the large terminal on the solenoid). Sounds to me like you have a bad set of contacts in that solenoid. If you hear a clunk and the starter does not spin that is usually the case. (The plunger is getting pulled back but the contacts are left in an open state for one reason or another) sometimes the copper disk gets eaten away from arcing or corrosion prevents a connection in the solenoid. In a rare case the windings in the starter motor itself can be burnt out, causing the motor to not move.

If you want to isolate the problem, use a jumper wire to jump from the large positive terminal to the ignition terminal. The starter should engage. If it does not, then the problem is in the starter/solenoid. Be sure that the Montero is in nuetral before attempting that one of course..