I'll start by apologizing if this is already common knowledge, but I couldn't find anything about it here, on other vw forums or on Google.

So we all know it's a pain getting an aftermarket amp or DSP running with the factory head unit due to it using can bus to turn on, and that using a couple hundred dollar high-low converter is largely the preferred option. I don't much like the idea of that, though I don't doubt they are doing a good job for many happy customers... But given that with vcds coding we can get a good clean low level signal straight out of say an RNS-510, using the 12v from the ciggy socket in the boot isn't such a bad option. But it only works with the ignition on, all the dash lights on, DRL and often auto headlights, and with the key out you can turn the head unit on for 30 minutes but won't get any sound.

What I've done is wire a cheap eBay 6v relay I had lying around (data sheet says it can run down to 4.5v iirc, and I got a firm click from mine connecting it to a <4.0 18650 battery from my ecig) and wired that to a male USB plug and connected that to the USB/MDI cable in my glovebox. The relay switches the (fused!) 12V feed from my battery to my dsp into its remote in terminal, and my amps are connected to the DSPs remote out terminal. You could still connect it directly to an amplifiers remote terminal if you don't have a DSP. Just be sure to have a suitably sized and positioned fuse for whatever 12v feed you use.

I haven't tried connecting a USB device to the MDI along with the relay (it's useless to me, no good for charging and no good for playing media) but I think there's a pretty good chance it wouldn't affect it so long as you splice it correctly. The relay coil draws something like 0.15w, and you could probably get something that draws even less.

So far it works great, though I notice when unlocking the car the amps turn on for a few seconds before putting the key in, but I have not had any issues with offencive pops from the speakers. YMMV. It can take about 10 seconds to shut off after turning off the head unit or pulling out the key, but otherwise it's doing a very good job.

No battery drain and it turns on when you turn the head unit on, what's not to like?