It sounds like a TJ, Your door switch sounds like its disconnected. I never really listen to my beeper, but I think its the seat belt reminder, Reach under your dash and feel the back of the switch, you should feel wires going to it, see if you can feel if there connected. If they are they could have gone bad or corroded.
I disconnected mine when I first bought the Jeep, I was sick of pulling the fuse every time I took my doors off like the dealership suggested. The wires are still there but I taped the ends and tucked them around some others so there out of the way.