http://www.subaru-svx.net/forum/showthread.php?t=57833
This may help. The plug in question is in the engine bay, and you could splice into it there. It is for the switch that is inside the transmission which turns on the reverse lights, so you first have to figure out which of the two wires is the switched one, and which supplies the power to the switch, then tap into the wire that's switched.
It might be easier to splice into it somewhere inside the car, but the only other places that I know would have a signal for when you're in reverse are the TCU, which I don't have a pinout handy of, and the reverse lights. I'm going to look around and see if I can find a pinout for the TCU.
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'94 Laguna Blue LSi ~159k
.......JDM ultra short-geared 3.900 STi Version 7
6-speed w/ Cobb shortshifter,
ECUtune 244,8.1mm/256,9.1mm i/e cams,
group N motor mounts, '97 grille, JDM clear corners, Momo JDM Legacy GT
steering wheel, apkarian's
LED tails,
silver STi BBS wheels,
PWR radiator,
redstuff pads f/r,
drilled/slotted rotors,
bontragerworks rsb #18, Koni/GC 450f/375r coilovers, Megan Racing adjustable
lateral links,
KMac c/c plates, Stebro exhaust,
ECUtune 1v5, Optima battery in the trunk where it
belongs.
Turbo project
'97 Ebony LSi ~137k #036
.......Power mode mod, JDM clear corners, BBS wheels.
AUX/pocket mod
Now a mod "over
there"
............Photo album