On Saturday, while driving back from visiting in-laws, I saw my battery charge light come on, and noticed a voltage drop on the voltmeter. So, Sunday morning, I took the truck to Autozone (which was the biggest mistake) to get the battery and charging system check, which they do for free. The guy told me my battery was good but low on charge, but didn't have me turn on the truck to check anything else. They also charged the battery for free, which took a little over 30 minutes. However, while putting the battery back in, and I was inside looking over their deals on air filters and oil change kits, the rep put it in backwards, and I didn't check after him. So I got in, and tried to start the truck, and got this obnoxious whine. Tried twice more, then got out and opened up the hood and told him the battery was backward. He fixed that, the truck started up fine, the manager came out, and they retested, this time telling me my alternator was bad. I was pissed, and was going to check out alternators at the carquest down the road. Got 20 seconds down the road, noticed my radio didn't work, and they replaced the fuse for me and it came right on.
So after all that, my questions are: How likely is it that this guy screwed up my alternator, bearing in mind that I was getting a charge warning light before? Second, if it is likely that they screwed up my alternator, what should be my course of action?
Thanks in advance for any help, I'm planning on talking to a mechanic too, but I wanted some outside opinions.