The three shortcuts are shown on the map below:
The Harding Truck Trail is yellow. The short-cuts are red.
The entrances to the short-cuts are not obvious, so you need to pay close attention. And even though the top short-cut is a maintained trail, both ends are difficult to find from the road.
I should mention that this is not a good route on a hot day. I personally wouldn't even consider doing it if forecasted temperatures were above 80 degrees.
Here are some pics from our hike:
Dave on the first short-cut.
Dave on the summit.
Santiago Peak as seen from Modjeska Peak
Heading down the top short-cut. You can see that this one is an actual trail.