sixtyfive c-10,
I think terminal #4 on your voltage regulator supplies ground to your idiot light when the key is switched on. This allows the light to illuminate. When your alternator begins to charge, terminal # 4 either becomes charged, or a relay opens...I don't remember which. In either case, this extinguishes the idiot light.
If your alternator checked bad at the store, you replaced it and the problems still exist, your first chore would be to make sure it is operating properly on the rig. The very simplest way to do this is with a voltmeter.
A battery reading is taken with the truck at rest. The engine is then started and the RPM's are increased. This should result in a increase on the voltmeter...high 13 volts or so.
If this all happens, it means the voltage regulator is telling the alternator to charge, but whatever internals connect to #4 are not functioning.
The odds would be that the voltage regulator itself is probably bad and the alternator won't charge properly.
The alternator worked at the store (hopefully)...will the alternator work on the truck. If it doesn't', it's the voltage regulator, or a broken or loose wire. The idiot light might be telling the truth.
Stuart
