Look from inside the trunk, you might even be able to see the old wire, use that to fish the new one through. If not, then you can use a big screwdriver, or what I like to use is a straightened hangar wire, with a hook on the end -- push that through the foam in the bottom of the back seat, fish the wire through.
I had to use the hangar wire trick to fish the wire from the hole near the latch to the left corner of the trunk. I pulled the wire out from one of the holes in the left corner of the trunk lid. then I used the hangar wire again to fish the wire from that corner to the bottom of the trunk lid, near the left hinge. So it took me two steps , a a little patience to get it through!