I had a factory installed Power Antenna on my old 93 Grand Prix. I replaced it about 4 times. Everytime the weather was going to be bad I would have to remember to leave the radio off. The ice would build up around it, if I forgot to clean it off before I started it up, would strip the gears on the cheap nylon cord inside. Or if during my trip the weather was good at start and then got bad enough to cake my antenna in ice as I traveled, I would have to remember to clean the ice off before I let the antenna down. All of that was a pain in the butt, I say stick with what you have, unless you are in area that gets no snow or ice what so ever. If you were me I would say It's not worth the trouble running wires, building a relay, perhaps redrilling the fender, mounting on the inside of the fender, removing the rust coat from your paint job and a estimated $150 to have to work for 8 months until winter rolls around again next year, and have to replace it. But that's my 2Cents.
Alot of new gm cars have the antennas mounted in the glass, there is a reason for that. Whether it's money because of cheaper install, or people like me begging for warranty work on their power antenna, who knows. My factory replacement unit cost me usually around $150 everytime, after one of those I was able to find just the mast and the nylon cord for about $40.00
I am so glad my new 2004 Grand Prix has the antenna built into the rear glass, and the reception is good too.