The front door speakers only work occasionally, what could the problem be?

This is an odd situation.  The front door speakers only seem to work when the outside temp is above 65 degrees.  They'll come on occasionally below 65 but shut off soon thereafter.  They also never work when it is raining or wet outside.  On a nice, sunny, warm day they work great, other than that I have to turn the volume up almost all the way to hear.  This has been racking my brain for a couple years now so any ideas are much appreciated.