Our SC's have dual antennas for better AM/FM reception as well as for the media content (ie. TRAF, text display, station names, etc). I've disconnected the connection from my rear antenna and still only pull FM stations from the windshield. I'm not sure which is specifically responsible for the text on the display (rear or windshield) b'c I still have the power harness connected to the rear antenna motor. Both antennas have dedicated inputs on the back of the OEM radio. The one that runs from the rear is the factory standard Motorola type antenna cable, while the windshield cable is smaller in diameter and thus Lexus proprietary specific (as I was told by a Toyota service manager). Here's a little something I wrote up... http://www.clublexus.com/forums/sc-4...ns-please.html. According to the schematics I've seen at Sewell the radio receiver that recognizes the remote lock and car key is mounted near the ignition under the center footwell. I don't think the windshield antenna has anything to do with the security/keyless entry.
Hahaha, Yea I get what you mean. I did a lot of research before disconnecting my antenna and adding a hardwired FM modulator that is always on. I had a fear that interrupting the rear antenna signal would have more consequences, but luckily both cables plug directly to the back of the radio and there was no major loss of reception b'c the front windshield antenna was still active.
I believe that it is called a diversity antenna when two antennas are used for the same signal. That setup allows the radio to choose the best received signal. (Inches different in location can make a big difference in RF reception.)