A buddy of mine has a 4age engine harness with the part number 82121-12400D which comes up as a 1986 AE86 GTS harness, The issue is that the harness is missing plug 3 for the ECU. It has plugs 1 and 2 but not 3. This clearly appears, to me at least, to be a USDM harness as it does have the AFM terminal connector. Now, according to the wiring diagram...the AFM wiring should run straight to the ECU as lines VC and VS. So, there's a AFM plug with wires that go to some mystery spot. This harness has not been cut. Is there a separate sub harness for that ECU plug 3 that we don't know about?

PICS of said harness

According to the information provided here...
http://club4ag.com/faq_and_tech_pages/E ... 04A-GE.htm
... you should have the 14-pin "C*" wire to wire (in-line) plug.
The wires that are connected by this plug are marked on the wiring diagram
with C* in a square box with two arrowheads on one side. Above the box will
be the number of the corresponding pin in the C* connector.

One side of the C* connector leads to the 14-pin "R" plug on the ECU. The other side
of the C* connector leads to the AFM, COR, check connector and check engine light.
So it sounds like you are on the money... there must be some kind of sub-harness
that pulls all that stuff together for feeding to the ECU plug.

Cheers... jondee86

Not sure about the USDM harness,
but on all the JDM harness I have seen the 3 ECU plug is actually part of the dash loom,
as they are the wires that go to the vehicle rather than the engine.