Well first I was going to tell you to clean your throttle body. But if the SES light came on right after the stereo was put in, it sounds like something else. Go to Autozone and pull the OBD code and see what it's telling you. It's hard to say what it could be without the code. Course you could always go back to whoever installed the stereo and tell them they screwed up your car. But that's not going to help much. Have you tried disconnecting the stereo to see if the idle gets better?
Chip
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