G’day, guy from Australia here.
I drive a Holden/Opel Astra H 2007 that has had an issue with what I believe to be a parasitic current draw. For your ease of potentially helping me out, here is some context:
Within the last 6 months I had installed an underseat aftermarket subwoofer with a built-in amplifier. The head unit is stock and does not come with an RCA output. I had to cut my rear speaker wires and run them through a line converter to plug into the subwoofer input. Power input for both the line converter and subwoofer are directly from the battery. Both come with provided fuses that have not blown. Grounding is done behind the seats under the torx bolts and bracket that hold the back seat in.
I have provided the ± wires to the line converter from both rear speaker outputs as input, but have not outputted a common ground. The line converter itself is grounded. I do not think this is important as it works properly though I will mention it in case it is.
Remote in (the 12v power signal that actually turns the line converter and subwoofer on) is provided from an add-a-fuse circuit in the engine bay fuse box. I used a multi-meter to find a fuse that only outputs 12v when the car is in ignition, which happens to be the Engine Control which is a 10A? (red) fuse. Add-a-fuse is a device that runs a normal fuse but also outputs a second signal which is also run through a fuse. Remote in is provided from the add-a-fuse to the line converter, which is then spliced and then run into the subwoofer so they both turn on with ignition.
I have a suspicion that the engine control fuse is not always off when ignition is off despite the fact that I have confirmed with a multi-meter straight after turning the car off. If I press the radio volume button to turn on the radio, it strangely turns on the line converter (my head unit contains an on-board computer that talks to the ECU) but not the subwoofer that is also attached to it. They both have power indicator lights. Is it possible that the engine fuse is running a low voltage to power some engine control circuits that is enough to power my puny line converter but not my subwoofer?
If anything is not clear what about I said I am happy to clarify, but that is the context and where I am up to at this current point in time.
Now, I had only owned the car for a year, and before purchase had been given a new battery, timing belt, etc. not very important other than the new battery. Some time up until a couple months ago, my car would become very difficult to start. I have a hidden diagnostics menu that shows me the current battery voltage, which would dip to as low as 10.4V sometimes. This is horrendously undercharged. I took it to my auto-electrician who performed a load-test on it. Test came back as rated for 599/600CCA which does not necessarily mean the battery is alive still. The problem was with the capacity which was being shortened significantly by being so deeply cycled many times.
A handful of jumpstarts in some very inconvenient locations later forced me to purchase a new battery against my family’s beliefs that the battery was still OK. By this point it would no longer hold charge when I turned the car off and would come up at no more than 7.0V the next morning. I splurged and got the best battery I could find which was 78Ah and rated for 750CCA. Problem solved. I hadn’t had any problems with slow cranks, low voltage, or anything of the sort up until about a week ago. I had parked my car at the airport for a week and did not leave any lights on as I had developed a paranoid habit after having my first battery killed.
Came back to find my brand new, top-of-the-line battery discharged to 8ish volts. I was shocked! I knew for certain now that something was slowly discharging my battery. Come today, my car is starting to struggling cranking over in the morning after less than 12 hours since its last use. Now I am here, writing this thread for you.
tl;dr: Wired up a line converter and subwoofer; cut head unit rear speaker wires to feed into line converter; remote in for both devices is being powered by a add-a-circuit from the “engine control” fuse that turns on with ignition and outputs 12V; turning on the radio while car is off turns on the line converter but not the subwoofer; I have killed a pretty decent battery by severely discharging it and am now doing the same thing to a brand new one; I am not 100% certain what is causing this.
My questions for you, generous reader! 
Is it definitely the line converter & subwoofer causing the current draw, provided I have not installed any other aftermarket electrical components?
I have only been jump-started since I left the airport a week ago and it has only had trouble cranking over once. Have I killed my brand new battery by severely discharging it?
Late edit to add additional question:
Is there a better fuse to switch the add-a-fuse to, or should I opt for something more simple like a switch that I turn on manually?
Thank you for considering my issue and reading this far if you did. Have a great day!