I do not know how to power down the empeg.

You pull the power from it. That's the only way. If you look in the kernel code, you'll see that it tries to do some things in the milliseconds after power is removed and power is gone.

There's no nice way to shut it down. Since power can be removed from it at any point, and usually is, there's no point in trying to define a nice shutdown. It has to do the right, most transparent thing in the worst case, which is the most common case, so there's no nicer way to shut down.

The only time this fails is when power gets removed while uploading data. But it should then be on AC, not in the car, so the changes of power going away happening are fairly slim.