I did this recently on my BT7, and all I can say is it's a pain in the you-know-what. The only way I could figure to get to it was to move that air tube, and that is NOT an easy job. By removing it from the front end, I was eventually able to stretch it enough to slide it down into the space between the inner and out fenders and get access to the relay box. It's not something you want to do more than once, so you may want to consider purchasing a new solid state unit. They look identical to the old relay box, but they are a lot more reliable.
Keith