Battery wont stay charged?

Just got myself a 94 Mustang GT, had a few problems but I figured I could fix them easily. One being the battery not staying charged. Thought maybe it was the alternator, went and bouight a new one replaced it and still no charged, I let it sit for about 20 25 minutes reving it everynow and then just to see if it was getting a charge and it just kept going down so now I'm stuck with a problem that I don't know much about, electrical wires. I have a wire diagram for it but I was reading it and trying to find the fuse to see if it was blown and I couldn't find it. Anyone know what I can do? Id rather not take it in an checked, but if I can't figure it out by Monday I'm going to have to. Any help would be great. Thanks.