Hmm, I'm not sure if I'm thinking of the same part, as I have an XR4ti over in the US. There is a "T" on the under side of the car, but I figure that'd be obivious.

If it's quite small, probably just a union. If it's larger, could be the location of the factory prop valve. On the XR though, the prop valve is just under the Master Cylindar and Brake booster.

Doubt that helps but it's worth a shot .