So now the question is again, is there 13v at the cables that come away from the battery, not the clamps, the cables, and what happens to that voltage when you hit the start button? If there is 13v at the cables and when you try to start, remains at 13v, then the next thing to check would be the starter solenoid. It should be at the left side of the battery and will have 2 heavy cables attached, one of which go back to the positive on the battery. There should be 13v on the battery side and nothing on the other side. Then there is also a small wire or two going to it and they would be the ones activated by the starter switch. When you push the starter switch it should activate the solenoid and there should now be 13v on both sides of it, if not, check to make sure you have power coming in on the one small wire and ground on the other (while pushing the switch) You might also find a fuse issue if you haven't upgraded the fuse box yet.