In the car's loom behind the head unit, there's no switched live for these modern units, the car's RED wire in the loom is live even when the ignitions off, if the RED wire from your head unit is attached to this, it will drain your battery in no time.
The head unit needs 2 lives to it, the units (not the cars) RED wire to a switched live (to power the unit) and the units YELLOW wire to a constant live (to store stations/settings). The unit should only work with the ignition switched on and store your stations when the igition is switched off.
If you're handy, you can check the drain with a Multimeter.
Turn everything off, ignition, lights, alarm, head unit, all that should be pulling power is the clock.
Disconnect the battery's negative terminal, set your multimeter to read Amps (make sure you know how to do this or you'll end up with a melted mess!) and remake the connection with the meter, ie one end on the - battery terminal and the other on the disconnected - cable.
Take a reading, something like 0.03 amps is ok, something like 0.40 you have a drain or you left something on, so check and read again.
Still 0.40? remove head unit and it's connections to the car and read the meter again, 0.03 now? it's the head unit that's pulling the power.
If you need to find a switched live for the RED wire from your head unit,
(notice I say units red wire, the wire coming from the unit, not the cars red wire in the loom, disconnect this end and tape it up)
flip out the plastic trim from around the gear lever, down to the left are the wires to the centre console, the ORANGE one (slightly thicker than the rest) is a switched live, run the head units RED wire to this ORANGE one (make sure the ignition is off first)
If you need to find a constant live for the head units YELLOW wire, use the wire you disconnected the head units RED wire from (you taped it up), that'll be a constant live.
Once you sorted the wires like this, recheck the battery with the meter, reading 0.03 now?
While you have the meter out, set it to read DC volts, replace the battery terminal, start the car and place it across the battery terminals, red to + black to -, the alternator should put out somewhere between 13.8 to 14.4 volts, if it's less than 13.8 you're a knackered alt.