Cransmash

I bought a 1997 1.6 EL 6 months ago. The CD/Radio is the original factory installed unit. I am unable to use it in cold weather because I get an incredible loud squealing feedback sound that comes from the rear speakers only. Only after my car has warmed up considerably does the stereo work normally. Anyone else had this problem? What is the cause? The Dealership wasn't much help to me, just told me to purchase a new stereo or look for a used one.