I had this happen on mine, too. Same year, same everything. First new vehicle I ever bought and I was poor then! My problem turned out to be the power door locks. They were shorting out due to moisture. Whenever it rained or the humidity got real high, they popped up and down like crazy. Sounds crazy too, even as I write this, but I sat there overnight to find this out. I finally had the solinoids(sp?) replaced and they were good for another few years. They finally stopped working again and I thought, Fine, I'll use the key (how 1990) and no more worries. I again had the battery draining issue and they said it was the door locks again! And it took a over week for them to figure it out. So I actually had them remove the solinoids and it never happened again. I actually sold it to my neighbor in 2002 and he wanted the locks back and the same thing happened! YMMV, and this may have absolutely nothing to do with your actual issue. It was really the only major issue I ever had with my trusty old Bronco II.