Hook up a current meter (ammeter) between a battery post and the cable that connects to it (i.e. you want the ammeter in series with the battery cable). Which cable? Makes no difference, power or ground cable works fine. Set meter on highest current setting and see what it reads. If more than about 30-50 milliamps (0.030-0.050 amps) you have some sort of current drain. Start pulling fuses and disconnecting circuits until you see the current drain fall off. When it doesm you have located the circuit that has the problem (it is whatever is connected to what you last disconnected). If the current drain falls but it still high, you may have multiple problems. Repeat the above steps until the current drain falls to an reasonably low level.