Audio Systems: new speaker static, olds alero, stock speakers

Question
I just installed new Jensen rear speakers in my car (Olds Alero). I now get an intermittant static/interferance from those speakers. I never got this w/ the stock speakers (which both died) nor am I getting the static in the front speakers. I have noticed it only seems to have static at mid to high volume levels (3 and up on a scale of 1 to 10). I can even hear the static when I change channels and it is silent for a moment. I've checked all connections and they appear to be good and I checked the antenna connection and it's OK. I also reversed to polarity in case I had the wrong pos. and neg., but it still had the static. Any help would be greatly appreciated. Thanks.

Answer
It sure does happen now, though...
The fact that both of the rear ones died may be your indicator, here, pointing towards the headunit.
Test the decks' rear outputs with a known good test speaker speaker and through short test wires.
If it is the deck, there is your problem. They are cheaper to replace than to repair in most cases.

Jm