Originally Posted by lancelucas
Many thanks for this, I got my sensor hooked up and working correctly tonight!
For search reference, I added a working rear diff temp sensor to my 04 WRX Wagon with 04 STI Cluster.
I found a harness from an 06 STi on eBay for $40 shipped. I simply searched for "sti fuel tank harness" and a few popped up. Rather than undo the entire harness I clipped the pigtail where it meets the next big junction (where the loom ends in your pic) and soldered/heatshrinked on some long extension 18ga wires with 1/4" wire loom to cover it with a few feet to spare after entering the cabin. I used the same grommet as the aforementioned fuel tank harness (pass side under the seat) as it had quite a bit of free space. I'll try to take a picture of the under-car routing next time it's in the air, it's late and I am tired.
I then crossed to the driver's side and I attached the ground side at a rear seat bolt. The cluster wire then went forwards up the door sill area like you did and into the cluster area. I had a spare dash harness so I stole the pins out and added one to B4 of my harness. A few more solder and heatshrink sessions, reassemble and I'm in business with a working cluster light! Thanks again for the info on this
Next up is DCCDPro with working cluster illumination and outside temperature sensor
Glad this has been helping people out. Yea way I did it is by far not the only way but if you copy it will work or just pull idea here or there. With DCCDPro you get get cluster lights and if you really want(I didn't do it) you can lock the diff automatically when rear diff light comes on just like OEM. Personally I would just lock it myself if it came to that, but option out there if somebody really wants to do it.
I wish I could take better pics to show you how cabling is route under the car but since I did this on jack stands and I have only a long lens for my slr camera its hard to do.