My car was suffering from the battery draining. Fitted a new battery, problem went away for 3 months but now battery draining again. No power to instrument panel at all. Put jump leads on and starts at 1st turn of the key

Best method is to connect a multimeter to the battery and look for current draw

If the current increases as you remove certain fuses then that is where the power is being consumed
Would first check the earth leads as common old older Nissan models for these to break and the alternator may be drawing from the battery