This is part of an unfinished How-To page for my webpage vanagoncafe.com. It should give you an idea how the cluster is put together.
That light panel has the previously mentioned Ox2 sensor light. The light is present in US and European spec T3's with petrol engines. There is small box connected between the cables that connect the speedometer to the transmissions in Syncros.
This box resides in the spare tire cavity. Every 60,000 miles, it triggers the light to illuminate. A reset button is provided at this box so you can reset the light. That is all it is -- a reminder light. When converting my diesel equipped Syncro, I discovered that the glow-plug indicator light occupies the same spot. It was pre-wired as to work as glowplug indicator as well. I only needed to swap the bulb from red to yellow to make it appear totally stock.
BenT