My 300 is the Racing model and does not have the estart. So I don't know what the battery box looks like.
The reason I installed that bar to the underside of seat is that on the Factory Replica parts diagram it shows this piece goes there. The bad thing is that the holes do now line up but I went ahead and drilled out my own holes and installed.
Why did you have to order a new rear tail light?
Here is a good way to prevent those rubber grommets from falling out.
Drill small holes just big enough for hand grip tie wire to slip through. Then tie the grommets to your radiators or guards (depending on where you have your grommets). - Problem Solved