I added another 12v outlet to my '92 legacy.  I ran a wire from the
battery to an in-line fuse to the outlet.  i placed the outlet in the
console by the handbrake directly between the two fron seats.  works
perfect!  I think running your own wires to the battery is a good idea
because you know you aren't fooling with any of the car's electronics.
it is a lot easier to do than it appears, i.e. there are grommets in
the firewall that will accomodate ther wire(s).

shaun

If you have a fuse box inside the passenger compartment, get
a meter and check.  If you want this always-on outlet for something
that's low power but might want to use an occasional high-power
accessory when you're in the car, consider adding a second
cigarette-type outlet that's always on and leaving the original
one alone.  

-DanD