1986-91 VW Vanagon Wiring to ECU Light in Instrument Cluster
This is written for an earlier model subaru engine.  Doublecheck your ecu references if your engine is late model.

Many of the convertees, including myself, are in the practice of using an existing wire to carry the ecu MIL light signal up from the ECU to the dash for placement of an engine code LED in the instrument cluster.  The easiest way to accomplish this is to use an already existing wire which is no longer necessary after the subaru engine is installed.  A perfect candidate for this is the yellow hi-pressure oil sensor wire (colours may differ on various vanagon years) which is in position TA-7 w/in the black junction box in the engine compartment.
To do so, you'll need to cut the wire (on the engine side of the barrel shaped connector) and splice it into the ecu MIL light from the subaru ecu.
At the dash, you'll need to sever the old hi pressure oil sensor yellow wire from the T14 connector which connects to the instrument cluster and feeds the blue circuit foil.  Once severed, the yellow wire (the end that goes back to the engine bay) can be routed to splice into the cut wire for the ox2 light.  

step by step:
At your engine bay junction box, find the TA-7 wire on the engine side of the TA round barrel connector.  Cut the wire and splice it to your subaru ecu wire that is designated as a feed for the ECU Mil light.
At your vanagon's instrument cluster, find the 14 pin ribbon connector and....
at a point about 2 inches out from the plug, cut the T14-12 hipressure oil circuit yellow wire.  
Now, at the instrument cluster ribbon connector, find the T14-14 OX2 connector wire.   Cut that wire at a point 2 inches out from the plug.
Splice the yellow hi pressure oil sensor wire coming from the junction box to the OX2 sensor wire going to your instrument cluster.
If you have a model of van that has no led for OX2, splice the yellow hi pressure oil sensor wire instead to the ground leg of a newly installed led which will serve as your check engine light.



Note:  Many vanagons also require that the yellow wire, on  the instrument cluster side of the cut wire at T14-12, be run to ground to prevent oil buzzers, flashing lights, and general mayhem.  To do so, splice the dangling end of the yellow wire at the point where you've severed it, splice in a brown wire and run the brown wire to a good ground point or splice into an existing ground wire.

Brent

T14 Ribbon Cable Connector

GRY-BLU	1	Lighting
BLU-WHT	2	HI Beam Indicator
BRN	3	Ground
RED	5	12v
YEL-RED	6	Coolant Gauge
VIO-BLK	7	Fuel Gauge
BLK	8	Ignition Switch 12v+ ???
GRN	9	Tachometer Signal
BLU-RED	10	Direction Indicator
BLU	11	Alternator Charge Indicator
YEL	12	Oil Pressure Control Unit (0.9bar)
BLU-BLK	13	Oil Pressure Control Unit (0.3bar)
BLU-BLK	14	OX2 Sensor (not after 87)
BLU-GRN	??	Coolant Low Level Indicator