1995 Pontiac Grand Am How do I fix my electrical car proble

Electrical problem
1995 Pontiac Grand Am Two Wheel Drive Automatic

I have a 1995 Pontiac Grand Am and the battery will not keep a charge.

I got the car for free and am trying to fix it up. We replaced some badly damaged hoses and bought a new battery for it.
It had been sitting in the owner’s front yard for more than a year, so it was kind of junky.

All of the interior lights work, the head lights work and it runs fairly well. I noticed that the interior lights one night were flickering on and off and so I go out and open and close all the doors (thinking that it might be the sensors) and went back inside. That happened for two nights and the next morning I tried to start the car and it didn't fire up.

I then recharged the battery and when I attached the wire to the positive post of the battery the car horn went wacko and then after a couple of tries it started to fade and eventually died while making some pretty weird noises (I checked the fuse and it is not blown). I asked a mechanic around here and he said that Pontiac's electrical systems go wacko when their batteries are low, but I had just recharged it. Anyway, I have kept the battery unhooked and have only connected it to drive it around once and awhile. While driving I noticed that the locking mechanism goes haywire (it will rapidly try to lock during the time that I am driving.) When you put it in drive it will lock and it seems like the problem might have something to do with that. I have checked the alternator and it is not bad so I am thinking I have a short somewhere, but I have no idea what to do to even figure out what or where the problem is. Also, I don't know if it makes a difference but the radio doesn't work and neither does its display.

Yesterday I was working on it and I noticed that when I am driving it, if I pull the door handle part of the way, but not enough to unlatch it, the interior lights will flicker on and the doors will lock. This happens every time I pull on the handle.
I ended up taking out the interior lights last night and the battery wasn’t drained today.