Interesting. I ran mine down the side of the tunnel on the floor as I wasn't 100% sure where it went. As mine is a LHD car, essentially the same as above but runs RH side in the boot.
Reminds me of an unfortunate story that confirms the loom routing I guess. A good friend got an eight track tape player installed in his Elan 'back in the day'. The installer ran four self tappers in to the top of the tunnel to mount the unit behind the seats. The car had untold mysterious 'electrical issues', until the owner realized it had damaged a bunch of the wiring!
One quick suggestion. If running speaker wires to the rear, route them on the opposite side of the car separate from the loom and well away from the main power cable.
Stu