Here is a picture I took from the floor of the wash looking up at the hill. The trail of wooden posts and concrete structure are visible in the upper left.
My guess is that its located at the red marker. It should be accessible via the Wilson Toll road shortcut trail.
In a screenshot from google street view at 213 N Altadena Dr. you can make out the posts across the canyon. If I can get ahold of a decent camera, Ill take some pictures from the road with much better definition.