We took the long way around, essentially following the 4wd trail to the west. There's another makeshift parking lot on the far side of the canyon, however, you'll probably want 4wd to get back up. It actually worked out in our favor taking a walk along the jeep road to the west in the am and through the canyon back to the parking lot as it got much warmer.
Took a quick day trip to Anza Borrego to a spot called "The Slot". While researching some extensive mud caves, I scanned over the area using Gaia Maps & Google Earth. It wasn't terribly difficult to find the turn off, although if you weren't looking for it specifically, you'd miss the spot. We headed east off Highway 78 and shortly after scissors junction, we turned north on Buttes Pass Rd. The road is a moderately traveled washboard road, but definitely doable for small passenger cars. When you arrive at the parking lot, locating the actual trail head is more difficult. As we entered the parking lot, we passed the new restrooms and parked along the right side near the trail head. There was a car parked there, so we ended up going the wrong way and missed the actual entrance to the slot canyon a few times.