The blowing the fuse on the step down makes total sense to what your saying as its lighting up the bonus lights aswell as everything else so i think your on the money there.
I didn't strip the bonus unit (in the main box) although i think i may of sanded all the points before putting it back together meaning i could of put something out of adjustment. I think it's still diming the lights but i can't get it to the point where i was before, like it's got worse.

If the bonus unit isnt adjusted right could it earth the lights dim, meaning it could be the bonus unit from the start?