What's this bit for?

[Eddleston]

What is meant to go in here?
Right in the middle of my rear bumper

There's something similar behind my bullbars at the front too...

[Albannach]

If you have an underslung spare, the winder goes through there. There shouldn't be one on the front!

[4thsurf]

Now you say you have something similar on the front? And bullbars (plural) meaning that there is more than one? Not that what I'm thinking you have I'd call a "Bullbar".

I suspect from your question and brief description that when your Surf was "Born" it may have had a factory fitted winch on it which usually came with a "Nudge bar" type arrangement and what ever else the "New car buyer" optioned up on top of that. winch seen here..

http://www.toyodiy.com/parts/p_J_199...html?hl=38389A

These things in the Antipodes are commonly known as a "Penn Fishing Reel" and I certainly wouldn't want to try and haul anything bigger than a pilchard in with one nor try and move a childs toy, By the time most got to our shores they were shot or the remotes were missing so got removed leaving you with what is now on the front of your Surf as the car dealers used the "Bar work" as a selling point