I LOVE CAR ELECTRICS! The multimeter shown in Petak's picture (or one like it) is an absolute must! You'll need to be able to measure amps (up to 10 A) and volts (up to 20) so buy one first!
O.K., here goes... If your battery is running down there are 2 possible reasons:
1/ Some electrical item is using up the electricity, or:
2/ The battery is knackered. To confirm this you need to do a "capacitance test" which shows how healthy the battery is.
(To eliminate no. 2, disconnect the battery and it it's still runs flat after 48 hours, you need a new battery!)
Assuming it's only no:1 above, proceed as follows:
1/ Having made quite sure everything in the car is switched off (including the radio) disconnect the "+" wire from the battery. Leave the "-" wire connected to the battery.
2/ Using your newly-bought meter, set the selector to AMPS and connect the wires to the meter as per the meter instructions (usually this means putting the RED lead to the AMPS plug and the BLACK lead to the COMMON plug, but not all meters are the same)
3/ Now connect the RED lead to the "+" terminal on the battery and the BLACK lead to the wire that had been connected to the battery's "+" terminal. You might find some crocodile clips useful to do this.
4/ The circuit is now complete and the meter will show how many amps are being used. The reading should be NIL or at a very low level. If you have a modern radio/CD player that has a back-up memory these use approximately half an amp on stand-by. Any reading over an amp or so is worrying and the cause needs to be found. Remove and then replace one fuse at a time and note the change in the reading, if any. If the removal of a specific fuse causes the amperage to drop then you have isolated where the power is leaking to. You then need to ascertain what items are controlled by that fuse and investigate further.
That's enough for the moment-report back to the forum with news....