I recently purchased an 03 CTS with about 80k+ mi. It has the standard bose radio that doesn't come in well in FM. AM & the CD player sound great. The FM has a hard time coming in, it sounds ok parked but could be better, especially the volume, it's a bit low. While driving it sounds like a far off broadcast. AM & CD, still ok. I took the radio out, antenna wire was fine. I bought a conversion antenna wire & tried using a regular whip type antenna & it seems to work a bit better, although I did not take it for a ride. There seems to be an antenna in the rear window above the defroster wires, which I believe is for the radio. There are 2 connectors leading to it from under the roof panel, I believe they go to a modulator for the radio. There is also another antenna about 6" long on a swivel glued to the outside passenger side rear window, an inside receiver is glued opposite from the outside one with an antenna wire leading off from it. Is this for the onstar? I also unhooked the 3 antenna wires just mentioned & the FM came in the same while parked or driving. Hmm? Oh, I've read issues with the rear defroster interfering with the antenna & there seems to be no problems between them. I guess, after all this blah blah, what it is I want to ask is, could it be the modulator?

 

I have similar issue. FM radio do not work if the outside temperature is high. like more then 75 degrees. AM and other things work fine.
When temperature drops FM radio starts working fine. I have 03 CTS base modle.