have either of you added anything electrical?, It might be hard to wire in a switch as for the cable and current demands of a car battery.
If you havnt added anything electrical, then I would prolly say that there is a fault somewhere with a factory electrical connection/device (power antenna maybe, those things suck)
Get a voltmeter/ammeter (current meter) can be had on any DMM from walmart ($15), can go through the fuse panel under the hood,
Pull all the fuses, Take the black lead from the DMM and put it to the big cable the feeds into fuse block (This is a hot 12v+ wire), then take the Red lead and put it to each fuse lead (point where fuse is inserted in the fuse box) you should eventually find the Amp drainage (the DMM will cause a short over what ever fuse you are metering enabling a connection) when you find the current, note what fuse it is, and it will give you a clue as to which device is causing it, then go and hunt from there
a quick tip, test the meter with know current draw before you try to find the leakage, For example, if you know that its a 8mA draw, then pull the main (100 Amp fuse) from the fuse box, and make sure you read that Current draw first, once you confirm that draw, then go accross all the othe sub fuses second, that way you know what to look for, and that you have the correct settings on the meter