There is a yellow wire with a red stripe that goes to the solenoid from the MPEM. There should only be 12V on that wire when the key is on the post and the start button is pressed. Disconnect the large red cable on the solenoid that goes to the starter so it does not continuously crank, put the 15A fuse back in and see if there is 12V on that yellow/red wire to the solenoid without the key. If there is, trace the wire back to the MPEM to see if somehow it is shorted to 12V. Check the connector to the rear electrical box where that yellow/red wire comes in for possible shorting of that wire to other 12V signals that exist on that connector. Look for corrosion or missing insulation. If it is the MPEM supplying the 12V to that wire without the key, then there is probably something wrong with the MPEM.