From the "clearing" at four corners there are four ways to go... 1. Continue uphill on Maple Springs Rd for 200 feet and turn right at the gate, which is Main Divide Rd heading south, or... 2. Harding Truck Trail
which turns right from the entrance to the "clearing" (the south-west edge for those more inclined), or... 3. Back down Maple Springs Rd where you came from. (then there's... 4. Instead of turning right at the gate you could continue straight, which is Main Divide Rd going north).
You want option #1. (Don't take option 2 or option 4! Option 2 will take you completely away from the peaks and eventually drop you at the head of Modjeska Canyon Rd many, many miles away. Option 4 will... well, I'm not really sure what option 4 does, so don't do it)
You'll know you're on Main Divide heading the right way because you'll start climbing a steep fire road covered with fist-sized loose stones that do nothing but irritate the heck out of you, not to mention slow you down. Don't lose heart on this stretch! You've come too far to turn back now! There'll be a little over a mile of this slog and you'll eventually level out at a flat spot, a wide left-hand switchback 400 feet below Modjeska peak, the lowest end of the "saddleback". Rest here. Get your spirit back by seeing how close Santiago peak is now. Take in the unbelievable views. From this point to the peak your breath will be taken many times. All of Southern California can be seen, from Big Bear to San Diego, Catalina Island to Banning, EVERYTHING. It's crazy.
From here the trail actually looses about 100 feet over the course of 3/4-mile as you make your way over to Santiago. Enjoy it while you can, it doesn't last. Around a left bend, turning you south, the steep fire road begins again. The rocks are still hard to deal with, but there are more lines through them than before. This is the final stretch. You'll make an uphill left switchback and then the trail follows a long crescent shape up right, around to the north face, wrapping around until you reach the peak facing south again.
You made it! You've climbed over 4000 feet!