I think we've seen two good options. A third would be to run a dedicated socket from the battery to the dashboard area. Shut the Positive Cable off with a Cutoff Switch, leaving the Brake Buddy line connected. BB recommends a direct line for many vehicles anyway, saying that in many cases the factory wiring doesn't deliver the amps needed for the unit to operate well. They'd probably mail you a kit to install one, but frankly, the socket is cheap.
I think if you wanted to do something like this and disconnect the negative battery cable (instead of switching the positive), you might have to actually remove both battery cables to be sure the operating Brake Buddy didn't feed back into the Focus' electrical system. Of course you could put switches into Both cables.