Try checking the amp draw, with the tractor off there should be no amp draw, if there is then you should be able to trace it down by un hooking things until it goes away.
If you don't have an amp meter you can use a test light between a battery cable and the battery post, no light = good, light = bad.
If you don't have an amp meter you can use a test light between a battery cable and the battery post, no light = good, light = bad.