Here is the procedure I use to isolate a battery drain:
Remove negative battery cable from the battery.
Using a 12-volt test light (make it up with alligator clips on the leads), hook one end to the negative battery post the other end to the negative battery cable you just disconnected. BTW - this procedure will be much easier if you use nice long leads (like 10 feet) for the test lamp. That way you can keep the lamp right near you as you work (more on that later).
The test light will glow or “light” if there is a flow of electrons through the battery.
If the “light or glow” is faint, that is normal. The battery is supposed to supply a minimal amount of power at all times to several components. For example the clock, radio or computer.
So, moving on, if the test lamp is bright, then there is a large drain. That is what is killing your battery overnight - and it needs to be corrected.
Now, with an eye always on the test lamp, start removing and replacing the fuses one by one. When the light goes out (or dims considerably); that will be the circuit with the drain.
WHEN YOU FIND THE AFFECTED CIRCUIT, LEAVE THE FUSE OUT.
Read the fuse block. If the circuit is providing power to stuff you can live without for a while (directionals, horn, radio, etc.) don’t do anything else right now. Just remove your test lamp rig, put the cable back on the battery and use the truck as normal.
See if the problem is solved. If the battery can now hold a charge overnight – well then, you have identified the problem circuit.
However, if it is a vital circuit (fuel pump, headlights or wipers), or you if want to fix it right away, go to the next step.
Now, once the circuit has been identified, comes the tough part. You need to identify whether the drain is in a component or somewhere in the connecting wires.
To find the ground (and trust me it will be a ground fault as opposed to a short circuit ). I say it will definitely be a ground because if it were a short circuit you would have had to deal with this long ago. At this rate of discharge, you would have most likely been blowing the fuse that feeds the circuit.
Back to the procedure.
Take the test rig that you disconnected from the battery post and cable clamp and remove the alligator clips from the ends of the leads. Take a couple of tiny male spade type solderless connections (Stakons) and crimp one to each of the two test lamp leads.
Plug these into the fuse holder just as if they were the prongs of a fuse.
If you don't feel like making one of theses rigs up, places like Autozone sell a special fuse that actually has leads that you can you can attach things to.
However you do this, make it up well and secure it tightly so it can’t get yanked out.
The test lamp should light up just as brightly as when it was attached to the battery.
Now look in your owners manual (sometimes it will give more detail than the fuse box cover) and see what things are driven by this circuit. Then, while dragging your test lamp around with you (see why I said to make it up with nice long leads) start disconnecting things one at a time.
As you do this, leave each component disconnected.
This is basically the same procedure as we used to find the faulty circuit – only now, we are now just dialing in as to exactly where within the circuit the problem lies.
Watch the lamp at all times. When you pull the connector off of the "problem child", the light will go noticeably dimmer - and you will have found the bad component.
Repair it or replace as necessary. Plug everything else back in too. Check your work with the lamp.
Okay, what if you disconnected everything single thing in the circuit and the lamp is still bright? Either your owners manual is not providing enough detail and there are other things still connected to the circuit or, and this sucks- it is in the wiring.
If it is still glowing brightly, it may be because there is still something plugged in that the manual (or fuse box) doesn't mention. Get the appropriate wiring diagram/schematics and make sure you have isolated every component from the circuit.
If it is in the wiring, again here is where the long leads come in handy. Pick one component and, starting from where it mounts, begin wiggling and shaking the wiring bundle (always watching the test lamp), working back towards the battery.
It will take some time but eventually you should see the bulb dim. Play around with the wires in that area, open the bundle carefully with a sharp knife, pull on them, twist them, do whatever you need to do to find the ground.
But always remember we are looking for a ground to the chassis not a short circuit between two wires. So pay special attention in areas where the wiring can touch the frame or where it passes through a tight penetration.
This sounds extremely involved, but really it isn't. And with any luck, these methods should find and repair your ground.
