1998 Ford Explorer Battery going dead

My daughter's 98 explorer's battery started going dead after sitting for a few days without being driven. We replaced the battery and thought all was fixed, but after a few weeks, it did it again. She had an amplifier installed when the car was new and we thought that it may be bad since the sound was cutting out. We had a new radio installed at Best Buy and had the old radio and amplifier removed. Again after a week or two, same problem. Sits for a short while and battery goes dead. She has an after market alarm and auto starter on the car too. Don't know if they can go bad and start acting up. Local mechanics don't seem to want to bother with it, so do you have any ideas for me as a novice mechanic, but concerned father? Thanks.