If you take a multimeter and touch one lead to the positive battery clamp at the positive terminal and the other to a bare metal spot or screw on the chassis in the trunk, what reading do you get?
What happens when you do the same thing from the positive battery clamp at the positive terminal to the negative terminal clamp?
What happens when you do the same from the positive clamp to where the wing nut at the end of the negative terminal wire bolts into the chassis?
I find it very, very unlikely that the main positive power cable has gone completely dead. This sounds like there's no ground to chassis.
The main positive cable from the battery, if I recall correctly, is routed just below the bottom sills beneath the bottom of the two doors on the side the battery is located in the trunk (and I think this is always the right side). How things "fan out" when it enters the cabin depends on whether you have an RHD or LHD configuration (again, if memory serves).
A battery isolation switch, if present, is very hard to miss. This, again, can have several configurations. Posting a photograph of the battery box and environs, preferably with the side carpeting that surrounds it removed, would be helpful if it's not a simple "something's not connected so that you have ground" fault.