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