Comment Scosche WIRED FM modulator (Score 2, Informative) 135
http://www.crutchfield.com/S-yd5TEirUa9W/cgi-bin/
The device wires in series with your car's antenna (a male and a female plug on the device) so that it doesn't have to send the signal wirelessly. Also, when the device is turned on it blocks the antenna for a large band of frequencies around the transmission frequency so there is NO interference. I'm having some slight gain problems, but it works so much better than those wireless transmitters. You need to wire it in to your car's 12 V and ground but I just picked up a cigarette lighter plug with power switch from Radio Shack and wired it to the device. As such, I ended up having to do no hardware modifications to the car, just get behind the deck to hook up the antenna jacks.