This one is driving me nuts. I had a decent JVC Car stereo. When it
starts getting cold here the reception of most AM stations gets
horrible - a high pitched whine accompanies most signals. Thinking
the JVC was at fault, I purchased a top of the line Pioneer unit. SAME
PROBLEM! It gets weirder. If I take a portable radio into my car,
with the car running, and the Pioneer turned off, no problem with the
signal on the portable. As soon as I turn on the Pioneer, the
portable makes the same high pitched whine!

To recap:
-No problems in the summer, only when it is cold outside!
-Taking a portable radio into the running car does not exhibit the
problem, so I don't believe that there is interference from the car.
-Turning on the car radio causes the exact same interference on a
portable radio in the car.
-I have tried 2 different brands and both had the problem.

Any help or ideas would be greatly appreciated.