I went to Peñón del Rosario in May. I used the information provided by OP plus the only other reference I could find on the internet, which wasn't too explicit.
The trail was pretty clear. We got around the first hill and reached the forest. We then met a guy with a reflective vest ("Park Volunteer" it read, in English) on a horse, who told us we were halfway to the top. We then crossed a glade and got into the forest again. The path was still clear enough, and as soon as it straightened northwards, there was barbed wire on both sides.
At one point me and my buddies thought that we had to start walking westwards, as we noticed that we were beginning to go round the Peñón. So we crossed the barbed wire and took a tiny lumbering road, but turned around after we noticed that we were heading closer to the completely vertical southern side of the rock, and that we weren't ascending anymore.
So we returned to the original path, which took us straight to the smaller rock which can be seen from town. My buddies were done, but I insisted on going to the Peñón. I thought that I could walk on the ridge and try to go round the rock to find a place to scramble to the top, but this proved more difficult than I had expected. The ridge was almost entirely covered in fallen trees overgrown with moss. As some of the trees seemed to have fallen years ago, the wood was pretty rotten and I fell through it a few times. The closer I came to the rock, the denser the vegetation became. There were also some spiny plants I fell on a few times.
After a number of unsuccessful attempts, I gave up and returned to the smaller rock. Here's a part of my Wikiloc route: http://i.imgur.com/ZXZ6YdO.jpg
I want to know where I went wrong. Should I have continued on the lumbering path to get around the rock, in order to climb it from the west side? Is the north side any better? Is it just impossible right now because of the vegetation?
I tried also on the 8th August but got chaised off the mountain by a thunderstorm. Nice hike through the forest, and fun going up the rocky top to get to the summit plateau. Beautiful day this time, clear skies.