On my 91', it was actually a bolt with a really skinny, weird shaped head underneath a plastic cap. The head of it was about one third the height of a normal bolt head, and almost Torx shaped. A 3/4 inch wrench KINDA fit on it, but the head was also tapered, wider at the top than the bottom, so rounding off the edges was easy.
It's designed to permanently attach to a square piece of plastic that fits in the pocket space on top of the bumper cap, the sides of the square plastic hit the sides of the pocket, thus holding the bolt from spinning.
Of course, the plastic broke off, so all I could do was bust the remaining plastic bit off of it and get a wrench on it. It SUCKED. But i was doing it outside in light rain at night with a flashlight. But I did have the bumper off, so I could see this whole thing better.
If your bumper caps haven't had that plastic thing break off the top bolt yet, you should be able to kind of get a socket onto the nut underneath and spin it off. If it is broken though, I'd personally pull the bumper. The working quarters are tight.