Guys, I hit the search first hoping to find an answer and nothing close to my year.

1996 Ram 2500 5.9 Cummins, 5spd.

Battery's kept draining down and as they were almost 9 years old I figured there's my problem. It was not!

Unhooked the right battery, put my test light in the left batteries ground circuit to check for draw and with the doors closed and everything off its lighting the test light. I started pulling the fuses out of the box beside battery and it didnt take long as the big square 50A Horn/Marker light made it go out. So, I opened the drivers door and proceeded to do the same and the IOD fuse did made the light go out.

Any ideas to rule out?

 

Since it's the IOD most likely a wire is grounding out under the dash.
First place to look for any electrical short problem is any aftermarket wiring.
Most problems are owner induced.

 

Sounds like something un-fused is grounding and killing your battery. Kind of like a large parasitic draw.

I'd feel around any wiring for hot wires as you could have an unwanted fire in that rig.

 

If you can get your hands on a quality ohm meter that reads 2 digits (0.00) to the right of the decimal point you can trace along the circuit and find the area with the least resistance. At that point you will be close to the short and can disconnect parts untill you find the culprit.

Without that you can just start disconnecting items until you find the problem on the circuit that the fuse supplies power to.

 

The IOD fuse supplies the radio for memory and clock so it will always have some draw. It could be something else on that circuit though, glove box light? not sure if the under hood light is on there as well. How long does it take to kill the batteries?