I have a 95 f-150 4.9.

Two days ago the wife goes to start her truck and its dead, I went out with my charger and it fired right up. All is well for two days. Then she comes home very upset, seems the truck wouldn't start, someone jumped it and she drove home. I went right out and tried to start it, nothing. Test the battery and it shows 12v. I first notice the negative cable clamp was loose, I take it off anyway, connect a test light between the (-) post and cable, I disconnect the hood light and made sure the door was closed, the light was dim and pulsating, so I proceed to remove every fuse to find what I believe could be a short, and nothing changes the light stays very dim and pulsating throughout the process? I connect the battery clamp and tighten it and the truck starts right up, no suprise to me, I thought the loose clamp was the problem but now I am wondering why the test light was very dim and pulsating. I completely disconnected all external accessories like amplifier and fan, even disconnected the alternator, still very dim and pulsating. Another thing I noticed if I open a door the light gets very bright, close it the light gets dim to the point of almost going out and pulsates. Now it gets weird IMO, I connect a mulimeter betweent the (-) post and cable, and it shows 12v's, the need does sweep 12 to 0 a couple times then remains at 12v, is this some sort of security thing with the memory functions? Is this due to the computer and radio memory functions? Needless to say, I am at a loss, any thoughts appreciated