Best bet, when you're dealing with weird old factory wiring, is run all new stuff.
Run a new main wire to the battery, 16awg or so, fused, from the battery into the (normally yellow) main power into the headunit.
The black earth wire can be bolted to any metal object on the chassis / bodywork. Often the headunit will earth through its mounting points as well.
The (normally red) acc wire on the heady should be hooked to a wire that is only on when the key is on. Acc wire etc.
After that run some brand new speaker cable from the white, grey, purple and green out to your speakers.
That pic that D posted up applies to the headunit colours, the car colours vary very greatly. I have yet to see a car that uses the same colour scheme as an aftermarket headunit.