Some circuits do have a constant draw, if there is a load that uses it, like radio settings, but it's not a battery pushing out that voltage. There is a load that is drawing that voltage. That's the one circuit I would not tap into, because it would be a 12v circuit.
If you tap it into / wire it to 12v battery it will always be on, unless you manually shut it off. This might make sense for you if you had a motion activation on it, and wanted it working while parked. It wouldn't kill your battery leaving it on for a work day, but there's a lot of potential to forget it and kill your battery.
You should find an accessory circuit to power it to. (It shouldn't draw that much or I would say wire it to the battery through a relay, and then just tap into an accessory for the relay trigger). If you wire it to accessory, it will be on the same time when your radio is on, (when the key is on, running, not when starting, when the key is back, and usually doesn't shut off until you open the doors.
Even if you got into an accident it would still record anything critical. This is also how I would do it because some are always recording and will only keep the last 5 or 10 minutes once you interact with it. It's more likely that if you can't physically interact with it, or if your airbags go off, that would also stop the accessory circuit, which should trigger to hold the recording.
