You can see on the far side that the "hell hole" around the battery has been previously worked on, but the rust was building up again on the battery holder itself. Time was right to get this fixed anyway.
In a clear case of "The solution is left to the reader", the Haynes manual says "Working logically, detach the electrical leads which go to the engine from the various looms." Uh yeah, thanks for all of the detail. There are 2 major wiring harnesses: the fuel injector computer and the high voltage to the starter motor on the underside of the engine. Here's the fuel injection computer harness: