[Can someone tell me what this is??]

Yes it is the internal temp sensor for the climate control, you're correct

[bubble in my front bumber]

......and there's the risk of the paint cracking as a result of the plastic expanding/contracting.

[RB20DET Gearbox Design]

I priced up the rebuild of the 3 litre box (which is similar to the RB20DET box from what i've been told) and it was about $480 for the synchro's and bearings from CBC. That's without gears.

[WTB: R31 GTS (274mm) Front Discs, calipers and struts.]

Preferably in Melbourne, am after the lot.

Cheers

[Ur Skyline - Stocker or Sreamer?]

Body - Stock.

Engine - Stock apart from pod filter.