OK, this is what I'm thinking, need some help because I haven't actually done this before.
If put a relay on the usb + wire I should be able to cut the power with the added benefit of preventing the thump when the car shuts off. However, if I understand relays properly, it needs constant power to hold a circuit closed. That's fine if I wanted to use the 12V for the relay, but what I wanted to do was use the ignition on the relay. That way when I shut the car down, the speaker shuts down before the dc-dc shuts down the computer. That way I get no thump and no hum. So,
1/ Is the ignition wire constant power that can hold the relay closed?
2/ Do I have to use the relay on both + and -, or can it just be +?
3/ I understand there are many types of relays, could someone point out the right one to use in a situation like this?