I'm going to have a stab in the dark and say that you may have cut into the rear loom which might kill some things but if you have something on permanently it sounds like the edge of the petrol tank might be cutting the loom but be in contact with and short circuiting some of the wires.
I would disconnect the battery asap and pull the tank out sharpish otherwise you might need to "throw another shrimp on the barby"
Apologies for the terrible, possibly racist, Paul Hogan/Fosters/Australian reference but I was trying to find a light-hearted way to say that you might have a fire on your hands if you're not careful.
I guess Pom is a polite name for an Englishman so I'm ready to take some gyp from down under.....