Afraid I can't help too much on the original question, though a thorough look at the body wiring diagram in the big fold-out pages at the back of the AE86 manual should help.

The rectangular plug behind the engine contains a bunch of the signals needed for the dash and stuff. From memory, it has one wire for dash temp guage, one wire for dash oil guage, two wires for reverse switch on gearbox and one wire for starter motor signal.

I remember someone posted this stuff before including a photo and wire colours, a search might turn it up.

Hen