Clearly the problem is a drain!

Something is drawing power than shouldn't be, to find out you have got to to put a meter in the line and measure the current draw, then the easiest thing to do is to remove and refit fuses one by one until the figure becomes normal, you can then isolate the source of the problem to that area.

A "Parasetic drain" figure is defined as just a few milliamps at the most and is normal, anything more is a drain!

You can't just simply put a voltmeter across the battery!

The classic as said is the boot light staying on, so check this first.

Martin