Posting some findings because I couldn't find this information anywhere on the internet.

Background:
Car: 1999 Subaru Impreza WRX STI GC8
Originally 5MT/Manual Transmission. '99 STI ECU and wiring harness.
Gearbox: 2008 Subaru Liberty H6 GT-B 6MT 6 Speed

Installed '08 Liberty/Legacy 6MT transmission into '99 GC8. New transmission doesn't have Vehicle Speed Sensor provision, end result is a GC8 without a functioning speedometer.

Product: DCCDPro VSSPro Vehicle Speed Sensor. Converts signal from ABS tone ring into a signal that the dash cluster can display.




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Short Version:

For '99/00 GC8, follow this diagram, however ignore the ABS signal wire instruction and try this instead:

2 core shielded wire

Black wire: Solder to GREEN wire on body harness side of ABS wiring in engine bay.

White wire: Earth/0V (crimp on ring terminal and bolt to car body)


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Long Version

Installed VSSPro as per online instructions. Wired red +12V wire and green ground wire into ECU wiring, connected black ABS signal wire to black wire at front left ABS harness and white ABS return signal/ground wire to green wire at front left ABS harness. White wire in 4 core cable was bolted to ecu mounting bracket, black wire in 4 core cable was connected to ECU VSS wire.

All I got was an ABS light and very rarely the speedo needle would spike, otherwise it sat on 0 km/h. I once had check engine light "33 - Vehicle Speed Sensor (VSS) or Circuit" after a longer drive, disappeared next time I drove.

Troubleshooting Attempts:

Unsoldered connections to ABS wiring, ABS light stayed on until I drove a short distance, then turned itself off.

Swapped out trimmer potentiometer with replacement (I thought I damaged it in unrelated incident) but no improvement.

Replaced VSSPro with another V5 unit installed in the same way. No improvement.

Tried switching black and white wires connecting to ABS wiring with no luck, still had ABS light and no speed signal.

Opened the VSSPro and measured outputs with a multimeter - power in measured steady +12.5V in accessory position / 14V with car running, ABS signal was steady around 0.5V, ECU signal fluctuated rapidly between +0.5 and +3.6V.

Disconnected red +12V wire from ECU power source. Ran +12V power lead from battery into a relay switched by the accessory +12V from the head unit. Red +12V wire in the 4 core cable was connected to switched outlet of relay. Connected green 0V/earth cable from 4 core cable directly to car body. Measured ECU output from unit while idling, measured at a steady 0.5V.

Attached white "ABS Ground/return signal" wire directly to car body and ABS light went away, but still no speed signal.

Solution:

Attached black ABS signal wire from unit to green wire at ABS harness. Attached white ABS signal wire from unit to car body (Earth/0V).

No ABS light, speedometer needle now moves along with car speed. Happy days

Useful Links & Images:

1999-2000 V5/V6 GC8 WRX ECU pinouts and reference table. Pins match up if you're looking at the plug from the side the wiring comes out of. Not the USDM N/A crap but an actual pinout for turbo models!







A'Pexi installation guide for some control unit for the various Subaru ECU pinouts to back this up. '99/00 WRX GC8 V5/V6 is circled. I wouldn't use the ground and ignition power displayed for the VSSPro.