I don't know about a battery isolator but I believe they wired it straight to the battery. There is no second battery. The hump is sealed and lined with polyfil. You can calculate the volume of the hump using something like styrofoam packing material. Fill the hump with the material, then pour it into a regular shaped box, then calculate the volume of the filled portion of the box (I was a stereo installer in another life). I do not remember the brand of woofer but once you have the volume calculated just spec one that will work within that volume.
Tweak