MTD lawn tractor won't start?

Hi, we have a MTD lawn tractor model no. 289707 w/ Briggs and Stratton engine. This spring my husband tried to start it and he got nothing, no trying to start just dead. It ran great last yr! So, he thought it might have been the starter motor and he was told to test it first by using jumper cables and when he did that it wanted to start. We figured it must be the starter switch, so we replaced that, nothing. Then he replaced the cylinoid, all the switches for the clutch, blade, etc. Still nothing!! Now he thinks it might be something to do with the electrical, a bad wire maybe? How do we check and find that? HELP!!!