There's also a good chance it's a grounding problem. Both the solenoid and The starter motor have no direct connection to the negative battery terminal - it has a positive lead, and grounds directly to the block.
The solenoid actually does two things - it triggers and moves the starter gear onto the flywheel via a fork (similar to the clutch fork actually), and simultaneously, the actuator rod also acts as a giant 90 amp relay switch, allowing current to pass through it to the motor. Most normal relays would not last very long under that much current, repeatedly. It all works because the flywheel and crankshaft and the rotating core in the motor aren't really grounded to the block.
Looking at the contacts on the starter, the small flat connector (the "S" terminal in the FSM) on the top is the positive terminal for the switch. The heavy gauge wire that hooks to the right side lug closest to the block (the "B" terminal") is the positive wire for the motor. The left terminal (the "M" terminal) is the pass-thru from the switch to the motor itself. The circuit is completed by the two bolts going into the block (make sure these aren't corroded)
Check the starter, and the wires going to the M terminal and the B terminal.
Check your engine compartment grounding points for corrosion or improper installation:
1) The passenger side right under the main battery box, to the fenderwell.
2) The square box just to the side of the battery.
3) Square box exactly opposite the one above on the driver's side, by the PS reservoir/ABS unit
4) The two wires going to the firewall, just to the driver's side of the VIN plate.
5) The transmission ground near the stock airbox and thermostat housing
Electrical stuff is weird - everything grounds to the chassis (it's a big source of free electrons), not really the battery. The battery and alternator negatives are grounded to the chassis so they aren't floating, not the other way around as you might think (everything negative grounded to the battery)
If you also happen to get really zapped everytime you step out of the car on a dry day, it could be a sign of a grounding problem, since the chassis is it's own isolated electrical system, and isn't grounded to "the ground".
If you're still confused, blame Benjamin Franklin for assigning "positive" and "negative" backwards - the role of electrons wasn't discovered until several hundred years later, at which point it was too late to change the system that everyone adopted. The two are *mostly* equivalent looking at the whole system, but because you're looking at everything "backwards", it's not quite intuitive.