Your pic in your post 19 looks like you have found the main earth on the transfer box. The other end of that lead should be on a terminal on the adjacent chassis (as per the diagram in my post 11, C01661). There should also be another ring terminal on the same chassis terminal, which then goes directly to the negative battery terminal. Check the battery terminal is secure. Then clean all the earth connections. Before disconnecting the battery, check your handbook for any disconnection procedure to avoid setting the alarm off. (usually involves ignition on to position 2 (ie all dash lights on but not to starter position), then off and disconnect the earth lead within 15 seconds. It may also require re-synchronising the key fob, again check your handbook. Incidentally, does the battery negative terminal get hot?