There are aerosol connector cleaners that do a fine job. As PCB mentioned, the factory strap type terminals commonly start to make faulty connections as they age. Replace them if corrosion begins to eat them away, and add a terminal shim if they don't tighten properly.
If you have a multimeter, set it to read DC voltage, and place one lead on the battery positive, then connect the other to the large wire on the alternator. The reading on the meter will indicate voltage lost in the circuit, and should be very close to zero.
If you find that wire to be losing some voltage, you can place the leads in different locations to pinpoint the location of high resistance. You can also do the same on the ground side, placing the leads on the alternator casing and negative battery terminal. It is best to connect them right to the battery post, doing that includes the battery connections in the test. If the reading changes much between the battery post and terminal, the connection is bad.
If those check out, you need to check the PCM control wire at the alternator plug, then follow it back through the harness, checking for breaks, chafing, or corrosion in the wire.
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