I just had a real close look at the first picture again. The yellow thing mith very well be a partially hidden yellow-green wire, which would mean there is actually a ground wire there! The hole in the wall loks like conduit too.
Our car has Japanese radio wiring... I remember lots of white wires with colored stripes. Never really tried to hook up a radio to the mess of wires hanging out of the hole in the dash board (my dad bought the car, a 1987 Mitsubishi L300 van used in 89, without the radio).
