Put an amp meter between the Battery terminal and the battery. Most multimeter have a 10 amp setting. See what your car is drawing when all systems are off. Amp draw should be very low, 50 miliamps or less for the pcm memory and stereo memory. If much more, you could have a circuit drawing power when car is off. Light circuits are a common problem. See if any lights are staying on. Glove box, etc.
If amp draw is low and battery going down quickly, it's probably the battery.
If amp draw higer than normal and lights aren't the cause. Start removing 1 fuse at a time from the car's 2 fuse boxes. When you remove a fuse and amp draw comes down to normal, you have found the offending circuit. BBB Industries- Premium Alternators, Starters, Power Steering Products
has wiring diagrams for most cars. Find out what the offending fuse powers and go through the wires and connectors listed in wiring diagram, until you find the power draw.
4th gen section has 94, 96 & 98 f-body service manuals for download. Each has full wiring diagrams. Also the link there will take you to 99-02 f-body wiring diagrams.
To make helping you easier, it's always best that you list make, model, year, and engine of your car.