Anthony, I replaced the starter on my jeep last year, a pig to get to. When I went to start it, the fuse blew for the ignition. Supplying company were little help, so out it came again. In this instance it was the terminal they added to make the jeep ignition feed plug and play, this was shorting against the terminal next to it. I had to remove The one from the old starter and all was well. Sounds like yours may not be that simple, but might be worth a look all the same, if your live is feeding back to the solenoid.
Alastair