battery, alternator or both?

Let me tell you what shes (2000 3.8 V6 Mustang) doin. . .When i cranked her up for to go on a night on the town everything was just fine. I rode around for about 10mins and notice my lights was getting dim everytime the bass come in on my radio. so i turned her off. when i did that i notice i didnt have any interior light. I left her for about a hour and a half. a buddy and i came back in his car to see if we could boost her off but my interior light came on when i opened the door. guess what she started without boosting her off. On the way home my "THEFT" light in the dash cluster kept flashing but my check engine light didnt come on. My voltage was normal and the hand wasnt jumpin or anything. I got up the next morning to see would she crank but she didnt make a sound. i raise my hood my battery had a little fluid on it and my hot battery cable had some corosion on it. MY OLD FAITHFUL "THEFT" LIGHT WAS FLASHING SORRY ABOUT THE LONG STORY

It might just be the corrosion. If you were able to drive it, I doubt its your alt. Start it up, take the battery cable off and see if she runs :)

the anti theft light should always be blinking

It could also just be some kind of relay in there. I had that happen one time.

Take some water and baking soda and a old tooth brush, Scrub all the corosion off, whipe the connections down. See if it starts. The alternator would be dead if the voltage meter is a zero, so it might be your battery. I'd take a battery from another car and hook it up to see if the battery you have is dead. If it starts up then your alternator is dead or it's just not charging.