Audio Systems: Car audio speakers effected by temperature, car audio speakers, pontiac sunfire

Question
This is a weird problem.  I have a 98 Pontiac Sunfire and the rear speakers only work when it's warm outside, or if sun shines on the car).  If it's colder outside, they do not work.  They sound OK when they work. I just replaced the CD/radio, hoping that was the problem but it didn't fix the speaker problem.  What would cause the speakers to be effected by the temperature?  Is there something in speakers that can be affected this way?

Answer
Most likely there is one speaker close to shorting out the voice coil, or is mounted with the speaker terminals so close to the metal of the car that when it's hot they short to metal, and not when cold. Also, speaker wires that are pinched under trim or screws can act this way.