Battery keeps discharging while sitting.

Two things:

1.) There is a FREE recall on Ford ignition switches. They overheat and sometimes catch fire.

Copy down you VIN number and call the Ford dealer nearest you. Tell them you want to see if the ignition switch recall for Fords & Mustangs was done on your car. They will ask for the VIN number & check it against the master database. If it isn't in the database, then you can schedule an appointment to have the switch replaced for free. The TSB is #95S28, and don't let them tell you that they don't do that anymore...

2.) Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.

The ideal method is to disconnect the positive terminal, and connect a Digital Multimeter (DVM) between the positive terminal on the battery and the positive cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.

replace the fuel pump relay under the driver seat. i had the same problem with my fuel pump not cutting off when the car was off. its like 8 or 9 bucks at an autoparts store

JRichker gave you some excellent advice. Although it does not seem the problem here, you could take your battery to an Autozone (or whatever your local autoparts chain is) and have them test the battery. Does not sound like the battery is your problem but it couldn't hurt.

If that's the case do what JRichker said- damn good method. I have an optima and it works great!!!

I had a 5.0 efi in a wagon. The alt would charge 14 volts. But it also had a dead short in it and it would drain the battery over night. took me awhile to figgure it out.