The bridge ground wire hole will have been drilled on an angle down from the top of the body into the cavity. You'll need to dig that filler out (carefully) and you should then be able to feed a wire down into the cavity from the top.
You need to position the bridge and drill the holes yourself once you have the neck attached and has measured out the scale length etc. DB posted a good guide with pics a while ago, I'll try to dig up the link: Bass bridge placement