Eric's Corvette Caper - S4-E22
Corrected entry: When Red tells Eric he can drive the car, and they are sitting in it together, Eric turns on the ignition and the stereo can be heard. That is how Red knows Eric drove it and grounds him for a month. But, earlier when Eric moved the car back into the garage before his dad came home, he would have heard the stereo blaring.
Correction: Hearing the radio blaring and having the presence of mind to turn it off are two different things. He was probably so used to the radio in his car that it just didn't register that it was on. Besides which, this is a character mistake, not necessarily a mistake for the show.
Zwn Annwn