Hmmm. I just pulled my dash top off to take a look. I see where the switched portion of the wire comes off the ignition switch and has a number of things already connected (led lights, etc). There happens to be an open connection there already (picture attached).
The speakers are 12v and 5 amps. If I wanted to tap directly into the switched side of the ignition switch could use the open connection. Are 5 amps too much to pull?
Alternatively, I could use Tinker's suggestion of the automotive relay such as:
https://a.co/d/6SnSsVM
In this case, I would run a line from the positive battery terminal with an in-line fuse to terminal 30 on the relay, terminal 85 to the ground wire, terminal 86 to the switched side of the ignition switch, and terminal 87 to the speakers. (diagram attached).
I do have a usb port that is connected to the switched side of the ignition but the speakers are designed to be connected directly to a motorcycle battery so I don't think tapping into that would be a good idea.
What do you think?