Draining battery

Okay, I installed a new stereo several months ago, everything worked great.

However now, my battery is draining overnight (new battery). I put a meter in line with the -ve terminal, and with my car turned off it was pulling ~3.8 Amps.

I pulled the fuses one by one, and it turns out it's the curcuit that my stereo is on that is causing the drain (circuit is labeled IOD). I removed the stereo (which appears to be working properly), and tried again, still had the ~3.8A drain.

I've tried the haynes manual and the owners manual, but I can't find what else might be on this circuit? Any suggestions as to how to proceed from here?

FYI It's a '98 Dodge Ram. Thanks in advance.

You say the circuit is drawing that kind of amperage with the new stereo removed?  So it is not the stereo drawing the current, although I suspect the new stereo was the root cause.  Assuming the new head unit has higher power than the original factory stereo, it is possible the additional current draw has partially melted a wire in your factory wiring harness without blowing a fuse, and that wire is now shorting to ground.  It probably has about 3 ohms resistance to ground (judging from you 3.8A current measurement) when it should have more than 10,000 ohms.

If I am right, this is a major PITA to find and repair.  You will need the factory wiring diagram for this truck, and a very good ohm meter (or better yet a hipot insulation resistance tester.)  Good luck!