The 2 pin black plug is the antenna connector, its got 2 pins because the u13 has the power antenna as well as an antenna wire built into the back windscreen, if you have a look you can see where the defogger part is and at the top of the windscreen theres another rectangular bit of wire that overlaps the defogger. You might also have noticed that when the defogger is on it doesn't defog the very top of the back windscreen
Anyway, the standard radios that came with the bluebirds take this plug, and I guess are smart enough to know when to use what antenna. My radio reception was really good when I had the stock radio in there.
When I put a new head unit in, I cut the 2 pin connector off and used a standard type antenna plug form jaycar. The only decision to make is which wire do you put it on, the big one or the skinny one?
I used the big wire, assuming that it would be the power antenna one. But now I'm not so sure, because my radio reception is actually pretty bad most of the time, so I'm thinking of taking the head unit back out and trying the other wire.
Try them both if you can, and find out which one gives you the best reception.