Here is something to try:
Take a picture or write down the size of all fuses in each position on the fuse block.
measure the charge of the battery at full charge with the engine shut off.
Power off everything on the truck. double check that the dome light is off.
remove all of the fuses.
Let the truck sit overnight, then measure the voltage across the battery. It should have very close to the original voltage value. If not, then you have eliminated about 90% of the wiring. A short of some sort exists between the battery and the fuse block. Fortunately, because you have eliminated 90% of the circuitry, the problem will be easy to find.
Now, select any particular fuse and insert it into the fuse block at it's correct position.
If that circuit is causing the battery to discharge (again leaving it overnight after a full charge), you have found the problem circuit!
If not, recharge the battery, plug in another fuse. and then check the voltage the following day.
repeat the above process until you either find the bad circuit or go crazy.
You can shorten the checking time by using an ammeter that can measure up to 15 amps at 12 volts. I may be asking you to use a large amp meter but if you use a small one you are liable to damage it by connecting it to a circuit drawing too much current for the meter. Larger is better. Instead of waiting for the battery to discharge after plugging in a fuse, connect the amp meter IN SERIES with the battery - not across the battery connectors. connect one side of the amp meter to the battery's positive post and the other to the cable that was connected to the post so that the current flows though the amp meter. Observe proper polarity as specified by the meter's instructions.
As you add fuses you will notice a sharp increase in current flow in the meter's reading when you plug in the defective circuit.
Finally go through the offending circuit until you find the problem. Since you have a Tracker, it might be difficult to get a copy of the wiring diagram. The Haynes or Chiltons manuals may have a wiring diagram in the back for early models of Trackers/Sidekicks. you can usually find a copy of either book at the local library.
I hope that this helps!