Hey,
I haven't put it on an SVX but we did put the Terra in a Legacy.  And yes we used the speed sensor.
It's very easy, once you know which wire to use and where the original sensor is.  
Terra has been working great so far, no problems at all.
It's your choice on what you want to do.
If you need help running the DSI box and connecting everything let me know
Cheers
Jake

I can try to help, how is that sound.
First try to find the speed sensor.  Somewhere on the transmision.  It could be either two or three wires.
What I did was I taped into both wires and then pulled them in to the cab and before we hooked anything up I took a volt meter (AC) and grounded one probe and took the other and conected it to one of the wires to see when I get voltage.  It increases with speed.  
I believe it turned out to be the brown with red stripes wire from the sensor, but don't quote me on that.
If you can easily remove or disconect the sensor then try to see where you get the pulse from that way.
then hook everything up to the DSI box. You will also need to insulate those connections very well so corrosion won't take a toll, that's what I've been told.  I used self stripping tap connectors and insulated them well, and so far everything is working like it should
You will need the +ve, -ve, input from the sensor, if it is 2 wire sensor then link the two terminals and one to pin 2 of Terratrip in the DSI box.
I would have to find the DSI diagram to see which pins but if you have it then you should be able to follow it easily.  I believe you need to worry about the basic wiring and one of the two that show for speed sensor connection, with the only difference between the two being the link, I believe.
If you have any further problems you can give me a shout and I can try to walk you through it.  I will be away till next week so probably the best would be to call me on my cell.
905 512 9870
Hope that helps.
Oh and by the way try not to play with the adjusting screw.  I heard that it can be hell once you play with it.  We connected everything and didn't even need to do anything.  
Jake

Success!
I found all the connections I needed without having to bring any new wires through the firewall!

The key wire is the Yellow-Red wire that goes into the ECU.  That wire carries the pulse from the speed sensor.
I also found a good 12 ga. power wire that is "on" all the time behind the kick plate cover on the passenger side in front of the door. There is also a matching black wire that is probably a good ground, but I chose not to use that. Instead I grounded to a bolt that fastens part of the dash support to the firewall.

My new baby just ran her first Winter Rally on Saturday night!  The Snowy Safari in Minden.  I drove a bit cautiously because I didn't want to scratch her beautiful pearly finish, but still finished 9th out of 31 entries in the Novice class.
She handled beautifully with new Nokian Hakkapeliita snow tires.
I had the Terratrip connected in time for the rally but it just wasn't reading any pulse.  The day after the rally I fiddled with the adjustment dial on the DSI and it instantly started working!
I'll be ready for the OWR on Feb. 15.

By the way, I heard some drivers complaining about their shifting arms getting tired!  I just smiled! The automatic was wonderful. I just slipped between 2 and 3 whenever appropriate - didn't spend much time in D at all.  

SVX makes a lovely rally car for this type of rally, but on faster, performance rallies, it would not be too good because of the 5" road clearance - it would bottom out too much.  I suppose a skidplate might help, but I still don't think it's worth the risk of cracking up the car.

This is the best way to enjoy winter weather. It changes your whole perception of snowy roads!  

Scott
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92 SVX LS-L Black on Pearl
02 Outback VDC Wintergreen

Subaru - It's All I'll Drive!