[OAC_Sparky]

Personally, I've found over half of battery drain problems (ie. overnight drainage) is from aftermarket equipment like alarms, radios, amplifiers, powered accessories, etc.

The best way I've found to pinpoint the problem is to get an ammeter (many multitesters have them built in), make sure it has about a 10A rating, and pull the positive side off the battery, place the ammeter between the battery and the positive lead, and check the current. It should be less than .5A (actually it should be a lot less than that, but a .5A is a definite problem.)

If it is over .5A, start pulling fuses. After each fuse, look at the ammeter and see if the current drops.

Remember: Stuff like power inverters draw a lot of power even when they're not carrying a load (plugged into the lighter but nothing plugged into the 110VAC side). Stereo amplifiers are the same, if the amp is direct wired and signal switched, the switch can malfunction and the amp just sits there drawing power. If you've hooked up aftermarket fogs/interior lights, etc with a relay and not have the relay power side on an ignition switched power supply, the relay will draw power even though the lights are not on.

That's a start, anyways.