Put 'em everywhere.
One in the wall over the engine hatch to plug your 12V cooler in to with a
constantly hot connection...  And to use for your 12V trouble lamp when
looking into the engine.

One in behind the glove box to keep your hand-held spot light plugged in
to, charging with a hot with ignition connection.

One in the sheet metal by the battery so your passengers can plug their
Game Boy/Walkman//whatever in with their car adapter...

Ummm...  Gotta be more fun things to plug in.  You can get dozens of
cigarette lighters from junk yards when you're there for the more important
things.