Keep having to replace battery - 1999 Ford Explorer

For a battery to go dead on you like that there must be something draining the battery while the vehicle is switched off. It could be something like a trunk light which is not switching off or a short across some circuit that does not show up on any light. The first and easiest test is to disconnect the battery negative cable whenever you park the vehicle overnight etc. and see if the battery still runs down.
As a more positive step and if this does not affect the alarm, disconnect the positive lead from the battery and wire a bulb like a tail light bulb, between the terminal and the removed lead. The bulb should light up. Make sure that everything on the vehicle is switched off, the bulb should still be lit. Remove and replace the fuses one by one until the bulb goes out. That is the circuit that is running your battery flat. Check all the components on that circuit until you get the light to stay out and your problem should be fixed.
Check your charging system. With the engine running at about 2000 rpm, a volt meter across the battery terminals should read around 14 to 14.5 volts and drop to about 13.8 volts with the engine at idle. If this does not happen, the charging system may be defective. Even with lights on the voltage should climb, but will drop lower at idle.