If someone has already figured this out, then my appologies for posting redundant information, but I couldn't find any fixes for this, only questions asking 'how to' and statements saying it can't be fixed, cause it's part of the STi's DCCD ciruit. Anyone who has swapped an STi gauge cluster into a WRX (or in my case a JDM 2004 STi cluster into a JDM 2002 STi) knows that one of the 'side effects' is the rear diff temp warning light does not go out. Some people have slipped a small piece of electrical tape under the overlay to block it, but I didn't want to do that, so some studying of the wiring diagrams and manual where in store for me. It turns out that the fix is simple , just ground the rear diff light terminal (B4 in i10 middle connector). The link below shows the rear diff temp light going on and off as I hold the wire to a ground and then remove it. http://member.newsguy.com/~gtfour/su..._rear_diff.avi
There is one catch. One my car (JDM 2002 STi), there wasn't a terminal in B4, so I either had to find a terminal from somewhere or look for another one that was not in use. I didn't bother with that route, and instead just remove the plastic cover at the back of the gauge, and then soldered a wire onto the leg for the B4 terminal, and ran this wire to a ground. WARNING: While the terminals on either side of B4 are both ground circuits, there are positive terminals in the i10 connector, so make sure your wire is soldered on properly. Also, try not to apply more heat than is necessary to the leg, or you may end up affecting the solder connection to the board. I also taped the wire to the back of the cluster so that any movement on the wire would not stress the solder joint.
As for where to ground the B4 wire, that's entirely up to you. I ran the wire down to the steering column where there was already a vacant hole for a 10mm bolt, and just added a ground eyelet and bolted it on there. There are other options, but please note I have not tried them. Splicing the B4 wire to the C6 wire (10 pin, single row, i11 connector), which is a ground, should also work. On my cluster, B27 and B28 where also grounds, but those may only be for the JDM models.
With this mod, the rear diff temp warning light will light up with ignition, and as soon as you start the car on, it will go out. I'm actually thinking about adding the rear diff temp switch. Why waste a perfectly good warning light, when it's as simple as running the same B4 wire to one terminal of the switch, and the other terminal to ground.