If your polarity is negative ground then there must be power getting to the frame for the tie down spark situation. The million dollar question is where is the path to ground??????????????????
Here is a way to do a process of elimination kinda fast.
Take the batt (+) cable loose from the batt.
Hook one end of a 12V test light to the loose batt cable and the other end to the batt (+) terminal.
If you gotta get a test light get a cheap one so you can throw it too.
If the test light is ON hooked up like this then you see the draw.
Disconnect items in the system one at a time until the light goes out. When this happens you have just found the path to ground and that is what needs fixed. It could be the starter, dynamo, regulator, a switch or yep that one year old batt.
Everything should be off for this process to work, key off and any electrical item turned off.
Good luck