engine idle rough and poor gas mileage on 1998 Cadillac Seville STS

dealer said I need new spark plug wires but they were replaced about 15k miles ago

Asked by for the 1998 Cadillac Seville STS
If the wires weren't factory, then they CAN fail in short order. Make the dealership guarantee their work, i.e. if the new wires don't fix the problem, you don't pay. If they balk at this, then they are guessing and take your car to a shop who will stand by their Diagnosis AND repair. I always do at my shop.