2003 Honda Civic Coupe power antena wire?

I've never heard of such a thing actually--I'm pretty sure all new hondas come with the built-in antennas.  The only thing I could think of might be a wire to turn on factory amps, but I didn't know they even made a civic with factory amps in it. 

Or maybe a actory antenna booster that needs to get a 12 volt signal to turn on and function properly.  But usually with vehicles that have those boosters, if it's not hooked up you will know right away because your reception will be horrible.

All 98-up hondas do not have power antennas and thus when you take a look at a typical 98-up honda wiring harness from metra (70-1721) or scosche, there is no blue wire.  What you might wanna do is take a look at this picture and kinda match it up with your factory plug, and see what wires may be remaining (whose function is still unaccounted for).  If you're left with just one or two wires it may very well be the hookup for an am booster. 

Easiest way to verify that is to try and hook up an aftermarket head unit (using an adapter like the one in that picture) and use the stock antenna plug.  If the aftermarket head unit gets barely any AM radio then one of those unaccounted for wires is probably the turn on for the AM booster.

If there's more like 3 wires that are unaccounted for and you have steering wheel audio controls, then that's probably what the wires are for.  But I don't think Civics have that as an option.  Also obviously if you have a factory-installed CD changer that is gonna complicate things as well.  Even though those usually just use a single DIN plug connection, sometimes setups with those factory changers will have additional wires in the harness.