Jeep cannot hold a charge

My 95 Jeep Grand Cherokee won't hold a charge on the battery. I just replaced the alternator and put in a new battery. The #21 fuse which is Security Module/Lamp Outage Module keep blowing in the fuse box no matter what I do. I got hit by another truck that knocked out my passenger side headlight, and I think that it may be draining the battery when the car is on, but I don't know for sure. What do you think the problem could be? When I replaced the battery the first time, I also accidentally hooked it up backwards as far as positive and negative go. It blew out a bunch of stuff. Do you think the battery gauge in the instrument panel is broken possibly as it only goes up about a quarter of the way?