This job is easier if you know how to do it. Not having done it before and not being very clever it took a while for the penny to drop with me.
Once you have removed the battery and box see the grommet, you'll see it has a number of nipples on it to route the wiring. Its difficult to get a wire through it. I know I tried for ages.
Its easy when you know how. Snip off the end of one of the nipples. Pass a long thin screw driver / bit of coat hanger through the grommet and from the other side tape the wire or a thin guide wire to the end and pull it through.