From the radio tower, follow a road to the northwest for approximately 1/2 mile. There is a trail that leaves the road at this point to your right, which is marked no motorized vehicles. Follow this trail to the northwest which will lead to a number of drainages.
All of the drainages lead to the first rappel point, the trick is to find the drainage that offers the path of least resistance. The closer you come to the rappel, you will find some of the drainages filled with brush. The drainage furthest west is just wide enough to pass through and will lead to a small pool (early April) at the bottom of a short down climb which is only a short distance to the first rappel point. Alternatively, there is a fin between drainages a bit further east that is an easy walk to the rappel point without the down climb. There are a number of other routes through the drainage that will lead to the rappel.
Whichever route is chosen, it should lead to a bolted rappel anchor above a steep drop off into the "hidden chamber". This rappel point is somewhat exposed, but not bad. A single 60 meter rope will take you to the bottom of this interesting chamber. From there, it is only a short distance to the top of the Morning Glory Arch. A tree above the arch serves as the rappel anchor for this double 60 meter rope rappel. This free hanging rappel is certainly the highlight of the excursion.
From the bottom of the Morning Glory Arch, follow the obvious trail out of Negro Bill Canyon to the canyon trailhead on Hwy 128.