Hmmm...

The empeg serial port provides power when the empeg is on - wire this to a relay which powers the wireless bridge, then the bridge is powered up whenever the empeg is on. Is this line high when the empeg is sleeping? Hopefully, but probably not. If it isn't, you need a way to prevent the empeg from sleeping when you turn the car off until it's sure you're not at home.

Perhaps a simpler solution would to have the empeg attempt to sync at 3AM every morning. Then you could simply get a timer which powered up the empeg and the bridge, and let it do it's thing. The only problem that arises is that if your sync takes longer than the timer allows, it'll get cut off. Will this leave a semi-working player in good enough shape to wait till it gets enough time the next night?

Those are some thoughts. I look forward to seeing how you actually do this, as it'd be a nice feature and I finally moved into a place with my own garage.

Matthew