I may have a place for you to look for that battery drain. When I got our 99, it sat in the garage for a little while with the battery that I bought the car with. I went in to use the car and the battery had drained very low and all I got were some clicking sounds. I then put a new battery after finding the existing battery was quite old. My daughter went to use the car one Saturday while my wife and I were out in her Camaro. The new battery was stone cold dead. Doing a search led me to looking at a couple of places where I could have a draw on the battery. One was the hood light. It wasn't there. The other was the glove box light. It was there! It seems the door on the glove box lost it's shape a little and the button that controlled the glove box light was not turning it off because the door was no longer pushing it in enough. A couple of stick on pads that are used under furniture legs did the trick.

Another thing is to use a Battery Tender and not a trickle charger. I used one over the last winter and it worked fine. It's best to just put a charge on the battery when it is low to bring it up which is what a tender will do. It is not recommended to keep a constant charge on the battery.

 

Not sure who taught me how to track down battery drains years ago, but I have used it a lot. A good decent circuit tester light will work. disconnect one of the battery cables either one + or - and put the test light between the battery and the battery cable should be a very dim light showing, the ecm and the am, fm radio, seats and hvac abs and clock have memories depending your model to feed very low amperage, it would take many months with a good battery to drain sitting idle. so place the test light between the battery and cable if it's very bright start pulling fuses one at a time and see which fuse changes it the most (less bright) thats the drawing circuit, I would check them all.