This huge wall extends from the end of the canyon (right at the gate where you have to pay a buck to enter the potrero) for more than a mile up the hill to the east.
It faces south, and is in the sun most of the day in the winter. Also, this area often has less fog/clouds then the north side of the canyon. If it's cloudy or even rainy at the campgrounds - don't give up until you know the weather over here.
Walk through the canyon, through the gate to the potrero, and find a red-painted rock path immediately on the left.