Hi. You were right in disconnecting the negative cable -- it's safer than disconnecting the positive, as it won't short if you bridge a tool to the chassis. Always disconnect the negative cable first, and reconnect the negative cable last. But that aside, let's try to help you....

Reading the voltage between the neg battery terminal and the neg cable just shows the voltage of the battery -- nothing more.
[And incidentally, a properly, fully charged battery should be around 12.65 volts -- a battery with nearer to 12.0 volts is actually nearly depleted.]

You haven't stated what the problem is, or why you're looking for a short. That would be useful to let us advise you more specifically. But without that info, there's one thing we can suggest concerning a "short".

What you might want to do, if you're looking for, e.g., an excessive drain that runs down your battery overnight, is to change the mode of your VOM from measuring volts to measuring amps, and then the following: first, shut everything off in your car. Then plug the VOM's red (+) wire to the 10-amp special socket of your VOM. Then connect the red (+) wire of your VOM to the negative cable, and the black (-) wire of your VOM to the negative battery post. Measure the amperage -- if it's too low to measure on the 10 amp scale, then reconnect the red (+) wire to the regular (Volts/Amps/Ohms) positive socket (usually, <500 milliamps), and then manually (if your VOM doesn't change scale automatically) switch to the 500 mA scale, and try measuring the amps. Successively switch to lower scales until you get a good measure of the amperage.

A car without a problem, with everything shut off (key out, no glovebox door open, or anything else) should draw about 30-50 milliamps across the battery and its cable. This drain is for only the radio's channel preset memory, the clock, and the car's computer memory (faults, etc.).

If, instead, you have a current of several hundred milliamps or more, this would indicate an abnormal condition, and could run down your battery (which, perhaps is your problem?). To proceed from that, I would hook the battery cable back up, and move to the car's fusebox. Successively remove each fuse and put your VOM (same mode and scale setting) wires across the fuse terminals to see which circuit is the high amperage culprit. Then, depending on which one it is, you'll have to go to each component on that circuit to do the same test, to see which is guilty.

If this doesn't help, or it's not what you're looking for, come back and explain what you are looking for.

Good luck.