One more thing....
When you do change the alternator, have the battery charged at the auto parts store, or charge it yourself if you have a home battery charger. If you don't charge the battery fully before you start the car once the new alternator is in, the alternator will have to work overtime to recharge the battery and also provide the power needed to run the car and doing this can immediately toast the alternator. Its called "alternator death" and its quite common.
If your home battery charger only has an amp gauge on it, leave it charging until the gauge reads between 0 and 2 amps. Charging a battery is like filling up a water tank, when the tank is empty, you can fill it at a fast rate, which is how a battery will charge when it is low, but as the tank fills, you have to slow the rate of fill or else it will overflow. The battery will accept less amperage as it gets closer to being fully charged.
Andy